Certain fighters ultilize knees on the mat and its a strong point of their game.
I think some fighters have been hamperd by not being able to use them in bouts.
Shogun, Vanderli, crocop are examples of guys who were very skilled in using them effectively and better than most doing so...since fighting in the ufc an not being able to use those weapons have hurt their game
Everyone could use them if allowed but certain fighters are better at it than others and I feel they have been stripped of one of their best weapons that they used for years and was a huge factor of winning a fight.
GSP
Fitch 
Stomps as well...Shit if GSP could throw knees to the head on the mat, im sure he wouldnt take it too a dec as he has been.