I believe whether a faction is playable or not is a reflection of how much time and effort CA have put into it. Playable factions are supposed to be better balanced, with a more varied and interesting unit roster, and get all the bells and whistles like their own intro movie and such.

I think it is deliberately designed to be very easy to mod the game to make all factions playable, hence why you don't need to unpack the game files to do it. It's kind of a disclaimer, saying "go ahead and play this faction if you want to, but don't complain to us if it turns out to be boring".