It's easy to say from the outside that the layoffs will hit underperformers and people who are just coasting, but the fact is that layoffs of this scale inevitably involve politics. We're talking about an organization that had so much trouble getting the Office people on the same page as the Mobile people that they launched a make-or-break product (Surface) with a buggy beta version of its killer feature (Office). You think that organizational dysfunction won't extend to the restructuring process itself?
At least if you do them and get them done quickly, the people you want to keep know where they stand.