Quoting Lucky Jack, reply 2
Since we can already scan our own planet today, I see no reason why any ship with even the minimum default scanners shouldn't be able to already support this idea.
Interesting. I just checked on GC2 and if you have reached an espionage level of "high" on the race owning the planet, you can see the planet's tiles and what has been built. I would think that with how we can scan the surface of our world from satellites today that it just seems wrong to not be able to.
Gameplay reasons? I guess you might choose to not attack a planet if certain buildings had been built, or maybe decide you needed a bigger fleet to conquer the planet.
Implementation cost? Less than having to be selective about which planets you are allowed to view.