[Psitticine]

I also think this is a rather good idea, but for slightly different reasons. I think it would encourage combined arms, just to make sure one isn't caught with one's codpiece down, as it were. That would mean units that might otherwise fall by the wayside would have more usefulness as recruiting them into existing armies would help make sure one always has at least some of the most potent counterforces available for whatever they run into.

I tend to agree with St. Patrik in that, during the early game, one goes up against the diverse independents, and that during the middle game, one couldn't over-specialize their troops with this kind of system without becoming more vulnerable. However, I do think that those are things that would go along with what I said above to help insure a useful place for all kinds of different troops.