I was hoping Sandy King would reply to this thread, as he is the developer of the Pyrocat family of developers. In his absence, here is a response he posted in a thread about staining developers and VC papers:
"There are several potential reasons to use a staining developer.
1. high acutance
2. grain masking
3. capability of building higher contrast for alternative processes
4. highlight compensation
The first three of this reasons are valid for all printing processes, graded and VC silver papers, and for alternative printing. Highlight compensation only applies to VC silver papers, and then only if you do not use a VC filter #3.5 or greater (or dial in the equivalent magenta filtration with a color head).
The original poster specified that he was NOT interested in results relating to VC papers, so only points 1-3 apply. I suspect that PMK and Pyrocat will be very similar regarding the first three points. (I am a long-time PMK user, and haven't experimented with Pyrocat. My experience is that PMK is high-acutance, and the grain-masking is effective for medium format and larger; I find PMK too grainy for 35mm. Many posts indicate that Pyrocat shares these characteristics, although I don't know about its use on 35mm. Also, one of the reasons for developing Pyrocat was the known oxidation problems of PMK in rotary processing, which is not recommended. )