It's not what he's good at that's bad for Microsoft. Microsoft stores will need to be more than data-driven: they'll need creative inspiration if they're going to help rebuild the brand, and that's a very different task from running Wal-Mart, even the entertainment division. It's not clear that Porter would know how to pull this off. Especially since he spent 25 years with Wal-Mart. This doesn't suggest a breadth of experience with retail experiences.
Incidentally, there used to be a Microsoft store at the Metreon in San Francisco. It was fairly swanky, but it didn't make me think any more highly of the brand or their products.