Interesting suggestion. The concept is quite contrary to the current one. I think this could have a huge impact on game mechanics and strategies. Also sounds quite hard to balance and to make obvious to the player. How much xp should a tower get for a stun... I actually like the last hit concept. Might be a bit unfair from time to time, bit its really plain and simple :-)
Ok you bring up some points like how much for a stun..what about this then so it's easier for you. When a creep dies it yields 100 percent xp. The tower that got the kill gets 50 percent. Any towers that did anything to the creep split the remaining 50 percent. So if 5 other towers did damage other than the last hit tower they each get 10 percent of the creeps xp. The concept of last hit is still prominent because you get the 50 percent whereas if it were say a challenge monster and every tower hit it that 50 percent last hit is still more than the 3 percent each tower would get. And it would make sense because unless it's an end game sat..it takes lot more than 1 tower to kill a challenge boss lol.