If they opended it up to ATI/AMD, more devs would use it, it would get more development and would probably contribute a lot to game evolution.
But for as long as it remains closed source, it will remain under utilised.
They did, AMD wasn't interested (tho I still say they should remove the limit on physx when a non-nVidia GPU is renderer even if its just on the beta drivers).
It would definitely contribute to game evolution tho if everyone could run hardware accelerated physics, theres so much more you can do when your not restricted to running physics on the CPU as a fallback - but no developer is going to cut out a significant portion of their potential audience making a game hardware physics only.
Because we all know spelling matters!