If the blade isn't making a dead square and parallel, then a sanding
disk on the same machine isn't likely to do any better.

Agreed. And I think you have to tilt the blade slightly for it to work properly, so you'll need to check for square.

On the other hand, if you need a disk sander and are willing to put up
with changing out the blade for a sanding disk every time you need to
sand something, this sort of setup works.

This is really only a "disc sander" in the sense that it's disc-shaped. In my mind they serve different functions. To me this is really an edge sander.

JP