First, remember that most people don't know who Paul Ryan is, and the ones that do know don't know very much about him.
Romney gets two benefits from picking Paul Ryan.
A. People who like Ryan (i.e., movement conservatives, William Saletan) will be more likely to vote for Romney.
B. Romney can run ads and do events that say, "Hey, look at this sexy new person on my ticket, who isn't me."
Obama also gets two benefits from Romney picking Paul Ryan.
A. People who don't like Ryan (liberals, some moderates) will be more likely to vote for Obama.
B. Obama can run ads that say, "Hey, Romney plans on getting rid of Medicare and giving you slowly shrinking vouchers instead."
Who would you rather be in this situation? Obama or Romney?