Along with the odor/irritation problem that some experience, there is also evidence that an alkaline-only process minimizes staining when selenium toning.
If you carry developer over into the fix you can get staining, especially as silver accumulates in the developer and fix. An acid stop bath puts a stop to that, so to speak.
With an alkaline fixer it is necessary for a really good rinse between the developer and the fix. It is a good idea to change the rinse water often or use a 2-bath rinse.
Another fix is to use the fixer one-shot.
For more on acid-free processing see Lloyd Erlick's web site.