Because it means that Harbaugh will run Denard out of town. Make no mistake, if Harbaugh asks Denard to switch positions, Denard is gone IMO. He wants to be a QB, and he's been a good one at that.
That means starting ANOTHER first year QB with NO PLAYING EXPERIENCE, who was recruited as a dual threat, but somehow will magically become Andrew Luck once Harbaugh arrives on campus.
Look at it this way, I'd rather have a flexible Hoke who will adapt to Denard than an inflexible Harbaugh who will run Denard out of town. Why? Because sudden change didn't work the first time and won't work now because the defense is so bereft of talent that's it's going to take anyone 2-3 years to slowly build it back up. In the meantime, we need an explosive offense, and Denard gives us the best chance for that IMO.