Muon2 of course sums it all up nicely, but the most compelling reason to pick Ryan is that Mittens endorsed the Ryan Plan, and Ryan can defend - or to put it more appositely - sell - the Ryan Plan far more effectively and compellingly than Mittens. Ryan ties the wonkish numbers bit with a passion about how while the medicine does not taste good, it is essential for your health, and that he really cares about your health, in a way Mittens is simply not capable of projecting. In a word, Mittens is uptight - and Ryan isn't.
When you get tied at the hip to someone else's policy, and the guy who formulated the policy is a good salesman, you hire the salesman. KISS.
Didn't you say Romney would have to be mad not to pick Portman?
