I suppose this could make sense from Ainge's perspective if: 1) he's convinced Rubio is a future all-star; 2) he's convinced that Rubio could competently lead a team right now; 3) he's got serious concerns about Rondo's attitude and shooting ability; and 4) he's convinced that Rondo is going to sign a huge deal in restricted free agency next summer.
Under those very specific circumstances, I can see the Celtics dealing Rondo for Rubio and probably an additional piece.
Tony + Rondo for Gay + Rubio? That's probably too much from Memphis' perspective.
My guess is Danny is throwing out crazy trade ideas like that with his inquiries. I doubt he is offering anything Memphis will strongly consider.
The other deal I think he might be throwing out there would be Allen (and perhaps a future #1) for Jaric, Darko, Buckley, and the #2 pick, hoping they will jump at getting rid of Jaric's salary, as well as adding a veteran presence like Allen.
I don't think Memphis would do it, but no one said the C's were making realistic offers, just that they made a pitch.