...and thus picking Rubio may have been useless..
A state-elected official will have more appeal than somebody who was elected and represents only 1/8 of their state. There is a reason Congressmen are rarely picked. They do not bring any real bumps to the ticket in the home-state and should only be picked to beef up some quality that the candidate lacks (see GWB picking Cheney).
I'm pretty sure Bush picked Cheney in the latter's capacity as former Secretary of Defense, not former Congressman from Wyoming or even former House Minority Whip.