If I had to make a guess, Romney went with Ryan to shore up the conservative base. He plays well to Republican voters, especially those from the Tea Party movement.
The problem is, I can't see Ryan drawing many votes from independents. Considering you need their support to carry an election, I think Romney made a misstep by not going after them. Plus, I don't think Ryan will help Romney turn the tide in any crucial swing states (especially Florida, where his budget plan probably isn't going to play well to the elderly population).
Ryan's no Palin, but this still wasn't his best choice. Anyway, I'm looking forward to the Presidential and Vice Presidential debates; they are going to be very interesting indeed.