I'm not sure if Stubbs is really a "top hitting prospect." I would consider him more of a "top fielding prospect" that isn't bad with the stick. I think he would provide value to the Reds this year with his glove, and at the same time get a feel for major league pitching. Like Bruce, he's going to have growing pains- probably much worse than Bruce. Like Bailey (and Bruce), I'd rather go through those now while you're not in contention vs. next season when you (hopefully) are- and the team needs you to step up and contribute in a major way.
Finally, and I acknowledge that this is very low on the priority list for reasons to promote Stubbs, but it does create an opening for Heisey, who certainly doesn't have anything left to prove in AA. That said, my hope is that they sell high and package Heisey at the deadline before promoting him (and possibly exposing a weakness.) If they don't move Heisey in the next month, he has to be in Louisville immediately- and it is becoming more apparent every day that he'll only get there once they promote Stubbs to Cincy.