[Nottom]

Think of it this way. Heads up 77 is a slight favorite against AK - true. But now add a 3rd player. Give him QT for example. Now 77 has to dodge an A,K,Q or T or hit his set to win. AK is still basically going to win if he hits an A or K most of the time so the added player doesn't hurt him as much.

Additionally, if you are playing 77 heads up against someone who raised preflop (with AK but you don't know that) and the flop comes 8 T J and he bets you can't really profitably call here even though you you are beating Mr. AK.

All the worring about there being a better chance of there being aces in the deck becasue people folded is pretty insignificant compared to the fact that you can't confidently bet 77 unless you spike your set.

** noticed the first part doesn't really apply to your post, but I think the second part is still semi-applicable **