each has their own rules, in pride you allowed to knee in the head but not elbow, in the ufc you can elbow but not knee. but as long as there some rules the more the sport will be legalized and grow. its not like them gloves provide alot of protection anyway.
gracie said in an interview afterwards that he would love to fight without gloves and knee and elbow to the head but yet he was whinning that he got kneed in the head. so why would he want to fight with knees to the head when its pretty obvoius he cant take them.
personally i think they shouldnt allow elbowing, because its to easy to cut the person and possibly end the career of a good fighter.
this here was from a knee