I do not think of it as a negative thing...
here:
It will be more like this:
Difficulty increase -> miners need more GHs -> more demand in GHs -> prices go up in CEX
Not this:
Difficulty increase -> miners panic-selling -> selling GHs CHEAP -> prices go down in CEX
Exactly !
Every diff, Miners need more GH/s and Cex.io haven't illimited GH/s to offer...