I believe Portnoy was running DT into the ground. I love songs like The Glass Prison, and Train of Thought is awesome. Some people think TOT was forced, but I think it sounds natural for them going way out of their element like that even though TGP was just as heavy, but back then Portnoy wasn't trying to be so METUL. With SC and BCSL, Portnoy's METUL influence sounded so force and with sections like "day after day", obnoxious. He was trying too hard. I realize there are four other guys in the band, but based on studio updates and even The SC documentary, it looked like MP was forcing what he wanted on the guys.
To answer the first question, no. I don't think he would contribute anything I'd like currently. I like all but one song on SC, but I still don't really have an urge to give it a listen that much. I only like 2 out of the 6 on BCSL, The Shattered Fortress and The Count of Tuscany. Even though Portnoy's try too hard tough guy vox are present on TSF, I really like the arrangement of the song and the concept of all the past riffs and melodies throughout the AA Suite. and I'm also one of the few who loves Jordan's solo. But I kind of wore those songs out a few years ago, and they just don't have the same replayability as classic DT. Again, not entirely MP's fault, but with his current musical taste at the time, he was taking the band into a direction that just wasn't them. Dream Theater play excellent metal music, but to me, the last two with Portnoy just sounded like they were trying way too hard to be balls to the wall.
The second question, yes and no. Mike Portnoy is and was a phenomenal drummer, and up to Octavarium, I loved him. Granted I didn't get into the band until after Score was released, but listening to all the albums, and usually paying attention to the drums, Mike was incredible. He didn't really have the same flare with SC and BCSL. We know now that he wanted to take a break as far back as SC or was feeling burnt out, so I guess that affected his drumming. He didn't win his usual drum award either. So I miss pre-SC Mike Portnoy, but I could do without current MP. I'm still up for an anniversary show, but to be back in the band permanently, no thanks. Even if I don't like DT12 that much, I still like Mangini in the band, even if ADTOE wasn't the most memorable either.