Really we have 2 questions here:
1. Why did Ben want to go with Locke?
2. Why did Locke take him?
---------------------------------------------------------------------------------
1. I think Ben wants to keep his "daughter" safe. And maybe he thinks Locke is the best chance the Island has of surviving
2.Locke probably thinks Ben has knowledge he needs to survive and keep the Island safe.
That said, I doubt they trust each other.
