carson's purpose may be to try and prevent blacks from voting in the democratic primary (and therefore give sanders, the "easier candidate", a boost), but it looks like it's doing more damage to the republicans than anything else.
the bottom three, together, would combine to second place. and i think that's very important to point out, because rubio is a sort of hybrid candidate - created through media obfuscation.
kasich is the sole remaining serious candidate. if he's not able to pull support from carson, soon, and bleed moderate support from trump, then the purpose of the primary becomes voting for trump to stop cruz - and trump wins. huge.