referrals are worthless at cex.io if they just sign-up and don`t buy GHs
I agree with you. You will get discount if your referral member buy GHs. So it is not the matter of only referral.
That is the way how referral system works on almost 99.9% of the sites.
You won't get anything from people registering, but only from people spending (on cloudmining sites), gambling (on gambling sites) or earning (on micro-work sites or faucets).
BUT not on freebitco.in faucet. You get 50% (If I remember right) every time he enter captcha and get btc.