I have a quite simple theory on this:
Lets face it, Romney is a smart man. What he is doing is seeing how the media reacts to Rubio and trying to get the left focused on him (Rubio) to see if they let anything out about him now. If all goes well, he will likely pick Rubio. If there are any problems, he will kick Rubio to the curb. He is just testing the waters, seeing how everyone reacts. It could also be a way of summing up support from the GOP and trying to captivate the Tea Party vote.
Ron will soon go one on one with Romney, Romney needs all the help he can get.
I think as it stands now, here are Romney's choices:
Rubio
Haley (SC)
Rand Paul
Chris Christie
Paul Ryan
Susan Martinez.
I will say that I would cross out Haley and Ryan. They have too much baggage for a run now.