Getawhale
EDIT: Actually here, let's get constructive. I feel a class limit of 2 would be a lot more reasonable, and feels just about right. I'm on the fence about whether to limit Medic/Heavy/Demo or whatever else to 1, but most 6v6 players I've spoken with agree class limit 1 is best for Medic at the very least. Having two completely destroys uber meta-game, uber counting, etc. But we have to assume Valve is trying to restrict as little as possible, and be ready for anything.
Having 2 medics won't work cause as you said it will really mess up the uber meta and for various other reasons, it would turn competitive tf2 into a totally different game that won't work at all. Having 2 demomen will be a real pain to play against even with the recent nerf, there was a time when there were 2 demomen in play and it was very quickly apparent that it wasn't going to work. As for heavy there was a time when 2 heavy's were in play and it was abused on last, pushing last is already incredibly difficult on a lot of maps and having push against 2 fully buffed heavys will literally be the most horrific thing to play against. Teams would run scouts and soldiers to mid and then when it comes to last they would just set up and abuse those class limits. Sure you could run snipers to try and pick off the heavy's but you can still play around that.