Don't get tied up in that whole roi thing when you think cex.io. Your not buying hardware, your just parking some BTC in a vehicle that pays a return. You can get that BTC back at any time. Depending on the GH/s market fluctuation you may get more BTC back or you may get less. I would rather have a few BTC in cex.io earning a small return than sitting in my wallet earning nothing. Bit like real cash sitting in a bank earning interest versus sitting in your real life wallet earning nothing.
Translation: Buy GH/s for more than itll ever make, resell it to someone who will buy GH/s for more than itll ever make. Ignore the depreciation due to difficulty.
The longest graph they present is one month. Perhaps that's why! If the GH/s value drops more than the rate of return over time then you are spot on, you would be better of leaving it in your wallet! 
All they have to do is wait until they have milked the next gen of cards for everything they can and set a few "monarchs" up for the hosted plans. Maybe that will happen by the end of next year. 
