I can't speak for others, but I know that my reasoning for making a deal with him was simply because we're clearly dealing with some supernatural shit, and so far none of our other friends have displayed any particular talent in that area, so the assistance of a trickster spirit will probably be useful until we can actually deal with whatever's happening.
We also don't really know the 'why' behind any of the supernatural stuff, so just being on his good side so that he explains some things will almost certainly be helpful. I realise we can't trust him as far as we can throw him, but I'd rather be stabbed by someone I suspect than be stabbed by someone I know nothing about.