I always thought that siege was kind of like RPG battles. Just my view of it I guess. I've already gone through and changed siege classes all to generic models and skins in a mod of mine.
Jedi being too weak is why I never liked having Jedi in siege. The reasoning that Jedi are not playable classes in Star Wars: Battlefront is because Jedi would be too strong. I agree to their reasoning, so I removed the Jedi class in the Hoth and Tatooine sieges for the mod I mentioned.
I think that it's not THAT critical if siege classes are able to be identified quickly... you can usually tell what class they are by what weapons they're carrying. Besides, how hard is it to distinguish red, blue, and yellow markings on droid armor anyway?
Now, about a probe droid class... it's probably possible with some vehicle experimenting. I recently made a useable turbolaser "vehicle" and it works like a charm (apart from some animation issues which can be fixed).