We've had two straight years of giving all the fall camp reps to a QB who gets hurt a few games into the season. It is reasonable to concerned about that happening again.
Personally, I'm in favor of taking the risk because of the possible reward, but I'm not 'baffled' by the opposing view. There is great risk with Taysom. In many ways giving Tanner an entire offseason as the #1 QB is a better idea.