Except than these 4 songs, I think they can replace any song in the set with any other song that they want. There's the second level of singles (One Step Closer, Faint, Given Up and Bleed It Out) that they played a lot, but I guess that's because these are fun to play.
So basically we have something like 8-9 constant songs from 22-25 songs in the set (and we have 3 sets), so they can play more non-singles if they want, but they're not doing it for some reason.
I don't think someone here ever said "That's okay that they're playing only singles". I think we all agree about that, some of us maybe want too much, but we all agree about the basic idea.