1) Kelly assumably brought in Book because in theory inside the red zone there are only short throws to be made, which Book is better at. This is an assumption. In reality this is stupid (which Kelly has a knack for) because ND went from one of the worst red zone offenses to one of the best with Wimbush, 14 rushing TD's. FOURTEEN! Who really cares how we score?
2) Also there is a big reason Book has a better comp %. One is they never let Wimbush throw on 1st down and continually ran that garbage QB read play that bad teams can stop by run blitzing without fear since we NEVER throw out of that play. Likewise this meant that Wimbush had to throw on 3rd and long ALL THE TIME. Many times forcing some throws in an effort to keep the drive alive. Book in his 1st start last year threw more times on 1st down than Wimbush did in the previous 3 combined. Not kidding. Zero throws on 1st down against UM ... ZERO .... who have top ranked D in nation and we still beat them with Wimbush. Also, completing a bunch of 3 yd passes improves the %. They never had Wimbush do that. I'd argue that Wimbush was a better deep thrower. Much stronger arm. Not even close really.
3) What metric? Wimbush is clearly the more dangerous runner and nothing scares defenses more. Tebow won a Natty without ever having to rely on throwing. One of scariest college QB's ever. Garbage in NFL but in college running QB's are very effective. Look at Ohio St. QB's, JT Barrett, Terrelle Pryor, Troy Smith, Braxton Miller, Cardale Jones. Great runners, garbage throwers, great college QB's, garbage NFL QB's. Top ten teams every year. The college game is way different. Urban Meyer would often call a QB sweep with these guys on 3rd and 6 and get it 9 out of 10 times. If he didn't he'd have 4th and one, then go for it and run the same play for a 1st down. Almost unstoppable. This doesn't mention that Wimbush is a far better deep thrower.
Not mentioned in comparisons is that Kelly flat out left Wimbush out to dry. He got screwed. They never set him up for success. Never ran offense to suit his skills and stuck him with that dam QB/read 90% of the time on 1st down and 50% of the time on 2nd down. PUKE! 2nd and 3rd and long all day. The offense Kelly runs for Book is a lot different. Book is a good QB, but he got a lot of more advantages from the coaches scheme wise that they NEVER gave Wimbush. He did what he did on athletic skills alone.