i've always had a tremendous amount of respect for Mike Portnoy. I learned how to sequence an album, how to make a setlist, how to play odd meters and metric modulations, and more simply how to really rock out with simple motifs and grooves.
I've always believed, and I still believe, that DT basically is a guitar-based group, and they became known because of John Petrucci's unbelievable guitar playing. But for them to gain a huge cult following and eventually become as well known as they are without any real commercial success is almost entirely Portnoy's doing. His connection with and true interest in the fans is unrivaled, and his persistence in the quest for making honest music, where the musicians are in control of the product, is admirable to say the least.
Drumming wise, yes he can be outplayed technically by these other guys. Petrucci can be out played technically by this guy:
https://www.youtube.com/watch?v=JslLRgpNhhc (I am NOT comparing the auditioning drummers to Marshall Harrison)
The truth is that Portnoy knows how to write awesome music and put on fantastic concerts with a great band of like-minded genius-level musicians.
That said, I think that Mike Mangini could really have a similarly effective rapport with the four other guys. It seems like Petrucci and Rudess have assumed leadership roles of a sort, so the new drum chair won't really need to fill Portnoy's shoes completely (and couldn't possibly).