I'll put my response in a multiple-choice format. If I have a toolbox, and that toolbox contains a hammer and a flat head screwdriver, then adding a Phillips head screwdriver will:
A) Allow me to throw away my hammer,
B) Cause me to throw away my Phillips head screwdriver and make do with the flat head,
C) Cause me to shop around for a cheaper flat head screwdriver,
D) Allow me always to have the right tool with me for the job at hand.
All three RBs have their benefits and their drawbacks; Bush's drawbacks may be more visible, but at least some of that is due to the scrutiny he undergoes (and has undergone since before he was drafted). Is Reggie more of a special-purpose player? Arguably, yes--but special-purpose is not the same as purposeless.