Just to be clear, I think Jordan joining the band was the greatest thing that could happen. Kevin and Derek didn't fit in with the other guys, who were all so vastly superior technically.
Have you seen When Dream and Day Reunites? Jordan and Derek trade off solos and Derek is IMO does a much better job. Derek also does a jawdropping solo on the last Ayreon album, that IMO tops anything I've heard Jordan do.
I don't really mind if they let Jordan do his keyboard solos, but I don't understand how it makes any sense that it's not the "role" of the bass to solo, but that it can be the role of the keyboard to solo. I honestly don't see the point in the keyboard solos, Jordan can also shred on guitar if he wants to, his keybaord solos, for the most, sound like watered down guitar solos transposed to keyboard. I'd rather hear more bass solos, Myung can pull off some ridiculous stuff that would sound so much more original than the usual Jordan stuff, and that wouldn't sound like a wannabe guitar solo.
In short, I don't have any problem with Jordan, except that he's taking too much space. And I'm sure that Derek would've been more appreciated had he stayed longer with DT. I think the only reason people don't like him is because they don't like FII (witch IMO has some of their best ever material), and because they haven't been exposed to enough bootlegs to realise that he was a much better performer than Kevin.
With that said, JMX still does a very good job with the role he's been given. Whoever doesn't hear him in the mix, has an earing problem or bad speakers.