I think Holder's effectiveness comes from his ability to throw an 80 mph 12-6 that might be one of the best in amateur baseball and then knock you out with a 95 mph fastball on the very next pitch, or vice versa.
I've seen him hit 96 a couple times. Once he's in the majors it's fair to say it might move up to 98 mph with some professional weight training.
You move him over to the starting rotation and ask him to throw 80 pitches a game your drawing away from his stamina. I don't think he can throw that many pitches and still have a his deadly fastball... so you're essentially getting a nice curve, a novice 3rd pitch, and perhaps a 90-92 mph fastball.
Holder is an all or nothing guy, which is what you ask out of a closer... not a starter.
As far as being a closer in the bigs, I think he has what it takes, more so than UCLA's closer's style of play. MLB closers rely heavily on the FB, and he has a great one. It's just overshadowed by his CB because college hitters can't do anything with it.