Awesome player, and he did some great things with DT ... but in the end, he wasn't the right guy. Jordan was the right guy; all they've done with him proves that.
I think being with DT helped Derek become a better musician (he even said something like this himself on the Score doc).
By contrast, I think JR helped the rest of DT become an even more awesome band.
Mostly this. The "right guy" thing has been debated forever, and will still be undecided when the Earth is cold and grey.
Had they kept Derek, it would have been a very different band, but I don't necessarily think that's a bad thing.
A Change of Seasons and
Falling Into Infinity are two of my favorite DT albums, so whatever came next, with Derek a bit more fully integrated into the songwriting and sound of the band, it could have been really great. We would've missed out on some of the great stuff they've done with Jordan, but in this hypothetical alternate reality, we'd never know that.
People who say "easily the worst" or "easily the weakest"... okay, maybe. But look at the competition. People still cry about Kevin Moore leaving, all these years later, and Jordan definitely has the greatest chops. How he chooses to apply them is still subject of much debate, though most would agree it's gotten better.
Black Clouds & Silver Linings has much more a feel of actual playing and composition and less total wankery.