I realized that I can charge cities population for building units -- it even seems this was considered at one point because at the top of the training window there is an "Available Population..." indicator.

I gave a brief test to draining population for trained units, but didn't take it very far beyond that.

I just wanted to know if anyone has any input on this.  For the devs, was this ever seriously considered, why was it scrapped, did the game or AI experience any massive issues with it?  For modders, has anyone ever tried it and given it a serious playtest?