But if ghash.io reaches the 51% it does not mean that they will start manipulating the blockchain for evil things.
1. Even at 40% they can start manipulating the blockchain for evil things.
2. Bitcoin's whole premise is that it is supposed to be decentralised. Very little trust required & also almost impossible for a third party to significantly hurt.
If you have to 'trust' that ghash.io won't use their hashing power for double spending, (and you can't even trust that because they already did Double Spend), then the whole security premise of Bitcoin is flawed.
The basic security premises of bitcoin concept was that there would be decentralized network of bitcoin nodes each with small share of hashing power to ensure that there is no single person or small group who can control Bitcoin network.