Originally Posted by Coldhardsteel
Mike Portnoy is a very famous and influential drummer. As you read in the quote from his facebook, he'd been working with DT for over twenty years, which is a long time for bands nowadays. He's also the leader and song-writer, so if he leaves, that's taking an amazing chunk of the band's spirit and idea with him and away from fans.
No Portnoy practically means no Dream Theater, at least to the fanbase.
Portnoy is the undisputed leader of the band. He co-produces all their albums, directs their DVD releases, writes a portion of the music and roughly 1/2 their lyrics, does a majority of the bands press, writes the bands set lists on tour, and just about everything else for the band. He is far more than just the drummer, or a 1/5th member of the band.