that would be cool, but it would also render the purpose of having different tribes useless, as eventually everyone (or at least all the big players) would be all 3 tribes.
People would simply begin chiefing tiny, inactive villages just for the purpose of gaining that tribe, and soon everybody would be every tribe and selecting a certain tribe at game start wouldnt really mean anything since you can be anything later on.
and if that were to happen, if anybody could make any troop from any tribe, the luster of having unique troops that only your tribe can make goes away since everyone can make them now...and therefore reduces the overall coolness of having tribes in the first place