The problem is in highlander and 6s the pyro literally rarely or even never ambush. Pyros in highlander only protects the medic, you may see big plays from the pyro succeeding in getting the enemy medic by kind-of-ambushing but its just rare enough to be just luck instead of strategy. And in 6s its only use is to deny enemy uber in a lame way, locking everyone in a corner by spamming m1. These just shows that the fire of pyro becomes kind of a cheap 60dmg instead of a weapon. And the reason to that is pyros fire should never be stronger than a heavy, which is hard to strike a balance. The attraction, instead of just making the fire more painful, gives a chance for the pyro to show its strength in dmg dealing.
marrrlowehaven't they already tried this though, and decided it wasn't good after playtesting it?
http://www.teamfortress.com/post.php?id=13542
The attraction force I think of is like one shot of airblast in reverse, including the efffectiveness declines from the peak at mid-range to negligable at high and short range, and it is a burst force so will not cause trouble to server smoothness I think?