I sincerely doubt ghash.io would destroy their own business by manipulating the market after reaching 51% of hashing power. It is likely they already have safety measures put in place.
And what if ghash.io (and/or other mining pools) is owned by The Fed, the NSA or another large entity that might like to destroy Bitcoin? What better way to do it than perform a 51 % attack, thus creating
a lot of FUD in the media and making sure Joe Blow stays far away from Bitcoin? The mainstream adoption phase would never be reached.
From Reddit:
If a 51% can occur, all trust in bitcoin should be lost forever. An investment that relies on people begging random strangers on reddit not to ruin it every couple of weeks is not really something that seems like a great thing to pour actual money into, tbqh.