I like the idea of bringing barbarians to your side, it's unique in the game. Converting cheap weak units is not a good trait, though, specially after the expansions.
What if defeating a barbarian camp was like the population ancient ruin?
My two cents:
"Defeating a barbarian camp has 50% chance of increasing the population by 1 in the nearest city and providing 50 gold. +25% production towards land units in every city."