Bitcoin is supposed to be trust-less. In essence, we are currently putting a LOT of trust in ghash for them to do the right thing. They have so far, but that's not the point - we shouldn't even be having this conversation.
I agree with this fella. Just because ghash.io hasn't done anything dishonest thus far doesn't mean it can't happen in the future. Plus, having this much hash power in one location should external influences try and take over ghash.io is still an overall threat to the network.