I think they might start Peterman for the same reason they started him last year- to see what he's got (in this case, to see if what he's got is better than last season).
There's no risk to letting Allen sit at the beginning of the season. As people said, there's a chance they both may play equally poorly against a top NFL defense, for different reasons.
Honestly, I think if Peterman starts, he'll look exactly like he did when he started last season. People are saying "everyone has a bad game." But that's different when you have a respectable body of regular-season work. (Peterman looked good last preseason, too.)
It may be the same situation as Peterman's start last year, but instead of Tyrod coming in for the 2nd half, it will be Allen.