To the Drinol posters...
Why don't we put in dinosaurs or blind wolves?
No.
Drinols were a scrapped idea and are unneccessary. Hunters took their place, so no.
Engineers are the only race that could be put in, unless they make a whole new one (like the Drones and Brutes). And, as previously stated, they are probably going to be in the game.
I wonder what they'd do? Repair Brute power armour? Repair your armour? Make arcade style upgrades (that'd be funny =))?
Maybe you could ride on them, sailing the skyies...
They are probably there for the looks =)