GHash does NOT worry me.
They would be stupid to risk a MULTI-million dollar businnes to a 51% attack or double spends when the price of BTC is so LOW (and I am saying this because the price of BTC is peanuts compared to what it will be by the end of the summer).
And they know that.
The mere concentration of hash-power harms the network.
They are going ahead and amassing >50% of the hash-power because they over-estimated the network hash-rate growth when ordering their miners. The reason is that
they at least want to make back their investment before the hash-rate climbs any more. This is short-sighted and will destroy their business. Much like the credit-default swaps that caused the 2008 banking collapse.