Constant Motion really does make the room throb live, though. They keep playing it because it gets a huge reaction. It's blisteringly speedy, heavy yet melodic, and very catchy, with a challenging solo section (which is probably tricky enough to still be interesting for the entire band to play).
How many times a tour do you see them, anywho? Some people may follow them from date to date (although I've never met any!), but the majority of people will only see the band a couple of times, max. Even if they have played it 50 times on a leg, any one person might've have seen it two or three times. For most people they'll either get it or they won't. And it's definitely one of those songs that's much stronger live than on the album.
I've seen Constant Motion twice. Across three gigs since SC was released.
I won't miss it if it disappears. I don't like getting repeat songs. I want to see as much DT as possible. Only time I've been happy to see a repeat song was In the Presence of Enemies pt. 1, for some reason. It was a really exciting opening track. But since my first gig (in October '05), I've seen Caught in a Web, Endless Sacrifice, Panic Attack, The Spirit Carries On, A Rite of Passage, Metropolis and Sacrificed Sons just as many times. As I Am once more. If anyone should be bothered, it's the band themselves. It'll only massively effect them and the bootleg collectors.
I'd like to see something else, sure. I don't think that they should have to keep it. But nor that there's any real need to get rid of it.