I completely agree that Symphony X stands on their own. As does Threshold, etc. No argument there at all.
But I don't think Dream Theater fans are always, or even mostly, the "guilty" ones as far as these comparisons go. Some threads are brought up by people not too familiar with either band and are simply wanting to get opinions, like the recent DT vs. Threshold thread in another forum. The name Dream Theater is everywhere and people get curious about other bands who get compared to them. I can go on e-bay and search for Dream Theater, and I'll wind up with a ton of entries that say something like "exciting prog disc from Threshold, similar to Dream Theater!" People pushing the DT name to sell their stuff, and it gets annoying. I would never tell someone that Threshold sounds like Dream Theater, because they don't! If I didn't know any better though I might think hmm, this Threshold band keeps coming up when I search for DT. They must sound like them, I wonder if they are as good?
Then there are people constantly comparing their favorite band to DT. I usually like to lurk somewhere before I post and I lurked on this forum for a while. From my observations there aren't too many Dream Theater fans coming in and saying that DT is better than SX. It seems that 9 times out of 10 DT is brought up so someone can say something like LaBrie sucks or make up silly puns on the name of their latest album. The rest of the time, the comparison threads are just as likely to be started by SX fans than DT fans, if not more often by SX fans. I agree the DT vs SX polls are kinda silly. (That Russ Allen vs. Bruce Dickinson sword contest thing was pretty funny though!)
And yes, there are the asshole Dream Theater fans. I read several of the DT forums and they usually have a lot of nice things to say about SX and there are certainly people there who consider SX their favorite band. They certainly have their arrogant fans, but it's really no different from any other band. The way some of those people put down other bands isn't any worse than the way some people in here put down the bands that they think are somehow inferior. It shouldn't be a competition!