Bryant
Mr. Sleepy
I think it's more of the fault on the band that forgot how to write parts that suit his vocals because they've become more interested in being a parody of themselves seeing who can jack their instruments off harder in 12 minutes rather than actually writing cohesive tunes.
While I wouldn't go as far as you put it, much of it has to do with songwriting and becoming more "progressive." If you listen to the newest Eden's Curse release, James is a guest vocalist and his voice sounds great in the context of the melodic metal style they write in. In fact, although the sample is fairly small, it is some of his best work in my ears.
Another good example of songwriting that doesn't work with the strengths of vocalists is the first redemption CD. Ray and Rick Mythiason (I know I spelled that wrong, but I am too lazy to look it up) sang on that disc. Both are excellent vocalists, but the songs really didn't "flow" the best in the world with either vocalist.
All musicians, no matter how talented are limited in what works for them, though some more than others. Unfortunately for vocalists, while the most expressive, are also the most limited in other ways. I think Dream Theater's writing style does not really use James' vocal strengths as a priority when writing songs. I could be dead wrong here, but I seriously doubt it. James has to work atround some really weird progressive music.
Bryant