Planetary Invasions

yes, provided there are enough resources

 But would it be worth the military and civilian casualties?

Depends on too many things to tell.

I think yes. Certainly better than destroying all infrastructure, resources and planetary environment through massive orbit bombardment, and ending up with your, but almost useless planet. Just destroy the military installations from orbit, and then invade with army and marines. If you have complete space and air superiority, the resistance wont last long, and you can capture civilian infrastructure relatively intact.

it depends on tactics of the invading army,and the weapons which they use.

The answer to your question is VERY context sensitive. For starters, from a modding standpoint, it's a yes&no. From a lore standpoint in Sins, then it really isn't very feasible.

The entirety of the Sins lore indicates that, while the Vasari did maintain standing groups of Shock Troopers (the unit organization is called a "Variment" in the game manual, IIRC), none of the races in the "era" that the events of Sins take place in have even marginal ground military capability.

Once again, there's a slight exception for the Vasari, as they've got Subversion, which is described lore-wise as espionage teams engaging in sabotage. But that's not actually ground military; that's espionage.

From a modding point, the devil would be in the effects. Code-wise, it's probably a pretty simple thing.

From an SF genre point, then it all depends on the SF universe in question, or even if we're talking about the plausible far future instead of a fictional setting; though that's difficult in any case, as exponential progress* means we probably won't recognize anything in the far future, or even the near midfuture. After all, while in Warhammer 40,000 or Star Wars it's perfectly feasible (and generally desirable in the case of 40K) to execute ground invasions of planets, in Star Trek, a planetary invasion wouldn't even be considered; not even the Klingons have sufficient military power to engage in a ground assault of the scale necessary to take control of a planet.

*It'd be hard for me to explain exponential progress for the most part; so I'll use an example:

It took probably around 2000 years before humanity stopped using archery and advanced into the gunpowder era. It took less than 50 years to go from unguided rocket artillery to intercontinental ballistic missiles. Do the math.

It's probably easier to just destroy the military installations on the planet's surface to force surrender rather than to haul in the necessary ground units to take the planet in a more conventional way, not to mention it can avoid additional losses to the attackers.

Infrastructure is easier to replace than personnel, after all.

It really depends on the weapons used. Invasions would be preferable to nuking the whole planet from orbit (TEC), but in case of more precise weapons without long-term negative effects (Vasari ion beam?), orbital bombardment is far efficient in dealing with hostile forces and preserving civilian infrastructure.

shotman is right,just look at the covenant in Halo,they use plasma bombardment,the targeted planet may or may not be re-colonized.

First, there's the feasibility of landing troops on the planet.  Compared to the cost of building a space fleet, planetary defenses (railguns, missiles, etc.) are pretty cheap.  Landing troops on a well defended planet would be very difficult, and if something happens to your fleet in orbit, then any troops on the surface are screwed.  However, this is not an issue if you have teleportation technology.

Also, even with a modern military, occupying a country or even a city is very difficult, and occupying an entire planet would be even worse.  

In my opinion, the best option to capture a planet is with a sort of planetary seige, consisting of pinpoint missile strikes on strategic targets (military installations, spaceports, factories, etc.) and a planetary blockade, starving the planet of resources until the surrender.  The only reason you would want troops on the surface is if you needed to extract something sensitive from the planet's surface, such as rescuing a spy or infiltrating a military installation to steal their battle plans.  

I'll just point out that the ease with which surface targets can be destroyed by orbital weapons HEAVILY depends on the weapons technology you use.

There's also the fact that, no matter what, invasions of ANY kind (whether bombardment or ground assault), are extremely tilted in the favor of the defender.

That's true. Vasari don't use "ion beams" per say; but it doesn't say what kind of beams they do use, so I suppose "ion" beams are as good a guess as any.

There's the slight discrepancy of the fact that if the Covenant REALLY intended to mass xenocide the human race, they'd have gone on a glassing spree.

I'll break this down by sections:

SECTION 1:

While you're correct in that planetary defenses are significantly cheaper than a space fleet, you've got 2 things wrong in your 1st paragraph:

1. You're assuming you'll be able to actually *land* troops in the first place. Generally speaking, any sensor system that can direct weapons fire against high orbit (est. 3000+ kilometers above a planetary surface, on average), said system can EASILY direct weapons fire against inbound, low-orbit landing craft.

2. Ground troops being screwed. This time the fallacy isn't in the assumption of troops being able to land, but in the fact that any sensible (or reasonably advanced) force would have included ground/sea/air support equipment, if at all possible. So ground forces aren't "screwed", per se, their just massively outnumbered.

It's quite likely that ground forces would be able to hold out for an extended period, depending on their equipment. It's also likely that an invasion force would have significantly higher quality equipment, but would suffer the drawback of significantly less quantity. Put another way, you could be looking at anywhere from 10-1000+ ground assets of any kind for every ground asset you've deployed (not including infantry, which would, IMO, be more on the order of 1000+:1 for defender:attacker ratios).

SECTION 2:

You're fallacy in the second paragraph is in assuming a force with the ability to invade a planet with ground troops would have parity with a modern military. This fallacy isn't nearly as bad if applied to the defenders, as it's quite plausible for current, modern equipment to be considered fairly "primitive" and cheap, inexpensive weapons if you compare them to a force equipped for planetary invasions.

Adding to that, it's more likely that, in a "plausible midfuture" scenario anyway, planets would be (relatively) sparsely populated, thus requiring only a (fairly) token force for occupation. To tell truth, from what I've researched on the subject, the general hard SF community's consensus is that troops would be useful for occupying territory, but not gaining control of it.

SECTION 3:

Here your fallacy is in assuming that a planet must rely on any outside resources for sustaining day to day life. It might require outside materials to sustain the quality of life its inhabitants might be used to, but probably not. And if you've got spies/infiltrators on the surface, then one of two things will happen:

1. You commence bombardment. The spies may or may not know this, but overall it's pretty irrelevant if they do. Essentially, the spies are expendable.

2. You simply wait for them to provoke enough turbulence that all you need to do is land troops in trading vessels. Docking personnel open up the bays and get a faceful of gun barrels.

Either way, IF, and a very big if at that, you can get personnel on the surface, then the target either isn't worth having or is so fractured that you don't even need full on military force to conquer them.

they tried their best

the only cases in which they chose not to glass worlds was when forerunner artifacts were found on its surface

 Yes, but the planetary bombardment we see in Sins is repairable,while Covenant planet glassing causes permament damage to the planetary surface.

There's one glaring problem with this assumption:

You assume that the techlevels of a race are largely stagnant; that is to say you assume that they won't (eventually) find a way around it.

Who's to say that the scale of the bombardments we see in Sins ISN'T equivalent to what we see the Covenant do in Halo? Personally I'm more inclined to believe several things:

1. Bombardments in Sins are anywhere from 10-1,000,000+x more destructive than the "glassings" of the Covenant.

2. Sins races have several ways to either reverse or mitigate damage; whether through "Trauma Nanomedice" (a Vasari tech) or simple having ridiculously robust bunkers.

3. Thus, the above would lead me to believe that Sins races are capable of widescale destruction, rivaling that of several popular scifi franchises (namely Halo, Star Trek, Battlestar Galactica, and Babylon 5), while also maintaining a parity with said destructive capability in the technology required to reverse the detrimental effects.