Well, now that I've had almost fifteen or so years to think about this (jeez, has it really been that long???), I think my thoughts regarding MP and his place in DT have solidified.
Obviously, first and foremost, he was one of my main draws to DT. His 'fan's fan' reputation was well-earned, and for a long time, MP.com was my internet home away from home for precisely that reason.
That all said, I thoroughly enjoyed ADTOE (consider a top-tier album and a return to form), so I'm not going to say that he was the integral magic that is missing from their most recent output. Hell, who's to say, if he'd stayed, DT wouldn't have continued on in a direction that fans hated?
I enjoy DT's last 4 albums, but as many others have noted, they are far from what I'd consider essential. One thing that I know, if it's MP's kit that we hear on these past few albums, I'd probably listen to them way more often.
There's something about Mangini's drum sound that just grates on me. The guy is a monster, and I suspect that he has little to do with the overall production choices, but I literally can only take so much of his snare before I start to feel fatigued. And I'm not just talking about DT12 (although, that record is pretty much un-listenable for me)
Contrast that with MP's drum sound, which I feel elevates virtually everything he appears on. Hell, even stock records like Adrenaline Mob benefited from MP's kit.
Flying Colors, Neal Morse, Winery Dogs–his sound has stayed consistent and his playing has remained musical throughout. Add his signature drum sound to AVFTTOTW, and I'd probably be listening to that album way more often.
Again, I think the band's albums all sound fine–this post isn't about them, but more about me and subjective tastes. And when it comes to drums, MP's playing and kit are that which I measure all other drummers against.