It's strange that MP is held to such a standard.
JM is a fantastic bassist. Hasn't really developed or done much in....decades. No one minds.
JLB hasn't changed his approach much or learned new and amazing techniques. No one minds.
I think it's just because when MM joined the band, one of the big things was that he was a teacher and so forth, and MP didn't practice much.
I think the problem here is that, because MP is so prolific, he's in A TON of records that all feature the same bag of tricks. He's playing in so many records/bands every year that you end up with a lot of samey sounding stuff, even though it's a different band playing. Couple that with the fact that he's in 3 bands with Neal Morse and 2 with Billy Sheehan... that blurs the lines even more.
Also, there's a big difference between playing your instrument and actually practicing. If you sit with your instrument and just noodle around or tackle a couple songs you know just to keep them fresh, you haven't practiced, you just played. Most real "pro" musicians, session guys and such, will tell you that real practice is needed if you want to get better at it. Practice is playing something you can't do untill you master it, that's where the phrase "if you are playing, it sounds good; if you're practicing, it sounds bad" comes in.
I have no doubt guys like JP, JM, JR and MM all PRACTICE their craft, and that's why they keep progressing as musicians. MP, on the other hand, seems to be the kind of guy who is more interested in getting more music out than actually crafting art, and that's why he basically comes up with drum parts in the moment and moves on to the next thing. Surely, the guy can play, but even he would tell you he hasn't properly practiced in years, maybe decades.