Well, you may be right. I think that there's a fine line to walk between boring people with the same riffs and song ideas on one side and changing the direction of the band on the other side.
If we are being honest, Images and Words is, by far, their most "different" album. They sound like a totally different band on that album than they do on the rest. Even their debut album, with a different singer, at least to me, sounds more akin to DT nowadays than I&W. And I love I&W, but it's more, umm, eclectic. They haven't done any other songs like "Another Day" or "Surrounded." "Learning to Live" is an awesome song, but way more a prog song with elements of metal than a prog metal song, if you know what I mean. Even "Pull Me Under" is pretty unlike their more current stuff.
I think this is why I could never go in on DT the same as others. I&W is their only album I actually enjoy listening to, and a lot of tunes have that feeling of a metal section vs a metal band playing other sections.
Karnivool are probably a good reference, they don't sound like they are a heavy progressive group unless they want you to. DT hit you over the head with it