The thing you have to like about Jimmer is that he improved TREMENDOUSLY every year that he has played so far. That shows work ethic, and demonstrates to me that if he has the ability to play defense he will. The combine measurements show that he has the basic tools to play defense in the NBA, at least enough to not embarass himself. He is strong enough to not be pushed around on a post up as well.
That said, I don't think he will last until 12. Jimmer will have a career in the NBA. I'm not sure I can say with any amount of confidence any of those projected past eight will. Some assuredly will, but I don't think anybody knows exactly who.
There is not a guard (who will still be available) other than Jimmer I would take a flyer on at 12. Take a big you like (I like Singleton), or try to trade down. Andrew Goudelock will be just as good as any of the guards later in the first and he can be had for a mid second-rounder (although I suspect he will rise.) If you really want a guard, hold on until next year when we will likely have two lottery picks to address that need in a guard rich environment.