It's strange that his voice is so divisive to people who call DT their favorite band. He's really integral to the sound . It's kind of like saying Aerosmith is my favorite band but I hate Steven Tyler. I guess it's possible but unlikely.
I have a hard time grasping this concept as well, but DT are very technical so Petrucci or Rudess' abilities are already enough to draw in people who are musicians or are really into the technical side of things. Whereas with Aerosmith it's all about a nice and good rock n' roll / blues song.
So in a sense I understand that for some people Petrucci, Portnoy or Rudess are already enough to like the band and don't like the vocals, but still... James IS the voice of Dream Theater. He's been there on every album except the debut, so whatever music exists from 1991 onwards, has been created for James' voice. There is no counterproof whatsoever that another singer would be fitting, because except the debut the songs have always been James' to sing.
To make another example... look, I totally and completely dislike Ozzy's voice, never liked him as a singer or even as a person, but he helped to shape the sound of Black Sabbath, and the only reason we're discussing Black Sabbath today is because Ozzy's voice was part of the first incarnation of the band that got famous. Sure a dime and dozen singers could have done a better job, but if we're even entertaining the possibility of thinking about it, it's because Ozzy and no one else was there at the beginning.
Same for James - I love his voice and while with my brain I understand why people may not like him, with my heart I just think a giant WTF whenever someone dislikes his voice. There's an alternate reality where DT never found James and either continued as an instrumental act, or found a random singer and stayed in relative obscurity for the rest of their carrer.