Personally, I at least want to hear one DT album without Portnoy before I decide weather or not I want him back in the band. While I obviously cannot know for certain, I strongly suspect that his tight control over the band is what led to the musical stagnation on the last 2 albums. His inability to let them just jam-out tunes like they did back in the 80's and 90's, and his insistence on creating a framework for each album, and to a lesser extent, for their entire career since SFAM, has no doubt sucked out a lot of the spontaneity in their songwriting. If you've seen the Making Of... DVD with Systematic Chaos, then you know that they tried to do an old school record with that album, but despite allowing some degree of free form composition, Mike still insisted on a theme for the album: it had to be "cutting" and "modern" and it had to have "balls", whatever the hell that means. Frankly, I didn't get the point of them doing another "metal" album when they had already done that with Train of Thought. Likewise, I didn't feel the need for another album of epics (a la Yes' Tales from Topographic Oceans) with Black Clouds when they had already done that with Six Degrees. The only real difference between those two new albums and the two older records was the self-imposed "need" to be modern with out of place hard core vox and metalish lyrics. Don't get me wrong, I love my shares of Painkillers and Holy Divers, but to hear the band that wrote songs like Learning To Live, A Change of Seasons, Lifting Shadows and Trial of Tears writing about Dark Masters and Counts of Tuscany just seems like a waste of talent (Speaking of which, I could go on about the under-utilized Myung, but it's already been brought up by others multiple times.)
On the other hand, though, it cannot be rationally denied that Mike did do the band A LOT of good in his day. A Change of Seasons aside, he really did step up to fill in the creative void after Kevin Moore left, which given the sentimental feelings most fans have for KM's compositions was a very gutsy move. He also did an excellent job of turning Dream Theater into one of the premier live acts of the last decade. I can't think of any other band that changes their setlist every night and makes sure every show has a healthy serving of deep cuts, not just the greatest hits repeated ad nauseum. And the ytsejam bootlegs are one of the coolest things I have ever seen a band do for their fans. Most artists just include a few b-sides on remasters and compilations, not many have the balls and honesty to open themselves up creatively and fully expose their artistic selves to their fans like MP did with Ytsejam records. For all that I have a very deep respect for Mike Portnoy.
I just wish we could have the best of both worlds: Mike Portnoy leading, not dominating, the band, and still giving the others a lot of freedom in writing music. I think the only time we had that balance was on SFAM, and the high regard in which most DT fans hold that record should speak volumes about the need for that balance.