I disagree, although you have a point.
If siege frigs were much less costly (supply point wise), wiping out planets would be too easy. Fleets in Sins move very slowly. A stack of fifteen to twenty siege frigs supported by eight anti-strikecraft frigs could be used to wipe half of the enemy's worlds before they had a chance to react and there wouldn't be any good defense against it (hangar craft would be demolished by flaks, missile platforms can be passed). I usually build two or three to make them follow my second fleet and kill the planets they may come upon, while my caps are being more useful elsewhere.
Colony frigs. I say this unit is fine, at least for Vasari. I, for one, dislike going Space Egg first. Many people do, but I don't simply because I think that it is too fragile as your flagship, as it has no self defense abilities (although a high level Egg has the best "useful-to-useless ability ratio" among the Vasari caps). Going with any of the five cap ships first should be a viable tactic, and a colony frigate is necessary for the other four. I like to begin with a Marauder, a Carrier (it is not so bad after all, if you understand the repair cloud's real purpose
) or the Kortul (Volatile Nanites and weapon jammin' ftw). I need that colony frig there. It's fine, if not as useful and bound to be seen in numbers as LRM frigs are.
Overseer. Now, what do you hit first in a huge fight? Cap ships, right? Simply because one enemy cap's ability can cause mayhem among your fleet unlike any other ships'. Vasari have no way of defending their caps, other than those cap ships' own abilities. Well, Overseers will make your cap survive a lot longer. Five Overseers can mitigate damage taken from a really big fleet's focus fire for half a minute or more. One Overseer is taking 125dps off your ship (250 HPs every two secs)! So I say it is useful, even if not for its second ability, which in turn IS meh (it only pisses off Vasari players who wanted to use stabiliser nodes to quickly intercept you from the other end of the system, but end up in phase space for 2 minutes instead of one). To actually kill your caps the enemy will have to intercept your Overseers first. But he also has to kill your subverters which just turned off half of his fleet.
If suddenly everything in the enemy fleet becomes your top priority target, you're screwed. Again, Overseers aren't to be used in spams, but they're cheap by themselves and are your only way of making your caps live longer, way longer. They are useful, although I typically experimented with returning armada recently and never had a supply point to spend on any ship, Overseers included.