Adam Warlock is barely used because there are virtually no other writers (apart from maybe Jonathan Hickman) that would be able to write him in a manner that does justice to what Jim Starlin accomplished with the character back in the 1970's.
I'd much rather Adam remain in limbo than to see him turned into a caricatured character clad in Mass Effect armor as is the case with Brian Bendis's take on Gamora in GOTG.