I am also questioning what cex.io does but many people are just going to learn it the hard way.
I see referral links everywhere, youtube, forums etc and I feel sorry for those who fall into this.
Many people think they can get rich quick and they have only themselves to blame for not doing research.

I visited cex.io from time to time to see how it does and I saw the drop from I believe 0.08btc/ghash to 0.03btc/ghash. It did not fall slowly, it dropped like a rock which could be a manipulation.
There are a lot of bagholders or should I say hashholders in cex.io which will not get their initial investment and I hope they learn from this. We are in very early stage of bitcoin, things are unregulated, and people are being scammed everyday. Nobody is going to save your assess if you lost your bitcoin to scammers. I am not saving cex.io is a scam but that place sure is fishy.

If you want to trade to increase your btc holdings, it does not make sense trading at cex.io. btc/ghash will only devalue over time, you might get lucky from time to time but once the next dump comes ready to say goodbye to your investment if you are not prepared.