I think I understand where Barry is coming from, but I think it's all relative. There are few earworms like I get from Jellyfish, but they're not Jellyfish. I listen to some other prog rock and "melodies" are like water in the desert, or oxygen on the moon: nonexistant.
I think there's a good mix with DT, or at least there was. I love how a melody comes out of nowhere - "Without love, without truth... there can be no turning back!" or the "All that we learn..." part of "Beyond This Life". For me, and I know this is my opinion only, James was always a key part of why I loved DT so much and what set them apart, and a good portion of THAT was the distinctive melody lines of the vocal, at least relative to other prog metal outfits.
For ME, the vocals are a big part of why SC and BC&SL don't hold par with what came before; it became more... not generic, nothing about DT is generic, but more "of a time". Images and Words came out in the thick of grunge, a movement that was so strong that (three or four years later) led even bands like Def Leppard (Slang) and Kiss (Carnival) put out "grunge" records. Yet, there's not a note of grunge on I&W. It was moving in the opposite direction. I'm a fan of Mike, but I think around that time of SC and BC&SL, he was starting to look at other bands, and guys like Lars and Charlie Benante, and wanted DT to be "his" version of that. It's hard to put into words, and I have zero proof, but it's a feeling I have.