The DVDs do not sell well anymore, but part of that (for me) is because there's really nothing worth buying. I don't want/need to see these guys play the songs exactly the same as they are played in the studio, with the sound quality being less full and the vocals being worse. Combine that with material that I think is the worst the band have ever done (The Astonishing), and that DVD would have been a hard-pass for me.
So what would I buy? Short of the performances surpassing their studio counterparts in some way, maybe I would be more interested if the performances were looser, included jams/extended parts, or were reworked live so that it was like listening to something new...
Their Tone changes live. It may be played at exact studio tempo speed, but it's still not the studio recording. The bands that use a lot of Backing Tracks are more studio than DT live. Periphery, Animals As Leaders, Devin Townsend (he's the one that relies the most on backing tracks, and look at how fantastic Order of Magnitude sounds without them) are great examples, they basically play and rely upon the click. Dream Theater, I doubt relies on the click as much, only for the backing vocals being on time, but the others have full on Bass/String/Keyboard tracks for the entire song.
Their stuff is compositionally complicated enough to play as it is, so them doing it perfectly as on the album, is a treat in my eyes. JLB withstanding, as the vocals in Dream Theater songs, are not as easy to sing, every night for about 3 hours. Also, the "tone" of Dream Theater, JP sees JLB as being an essential part of. MP didn't and wanted a different singer or a different lead tone for Dream Theater.
But you are free to enjoy live music that way, I just find it an odd way to look at a live show. Reminds me of The Simpsons...
Lisa Simpson: "You have to listen to the notes she's not playing."
Snooty Man: "Pff. I can do that at home."