I actually disagree a bit. As much as I like when bands change their set lists a lot, Sonic Youth always used to have pretty cool standard set openers/closers + encore openers for many tours. "Shoot" as an opener in '92, "JC" as the first song of the encore -- what could be more awesome? Even if it became predictable, that's still deadly. "Sympathy for the Strawberry" as a closer of most of the '02 sets, with 'Disconnection Notice' as the first song of the encore. Closing the sets with 'NYC Ghosts' and then opening the encore with 'Lightnin' was cruel, but hilarious. In '98 they'd close w/ 'Heather Angel' then open the encore with 'Stil' (fucking awesome, again). These are all gambles to some degree, because an encore is surely when you expect to hear a 'classic', but the ones I've listed here (and others) were all pretty much played like clockwork. While it may seem like a drag to have these kind of predictable entries in every set if you're following them on tour, most people only see one or two shows, so there's a lot to be said for there being a static structure to a tour's set list, if that's what the band wants to do.
Well that was a long convoluted way to get to a point I'm not even sure I can make... the example you gave, where the encores actually aren't set in stone, has less of an impact for me (as an admirer of set list design, not necessarily as a fan attending multiple shows). If there's no one song from the new record that can serve as a great encore opener, then it's better off to just get the new material out of the way in the primary set and save the encore for classics. Otherwise, how exciting is an encore that starts with "Lights Out" or "Thunderclap"? If I weren't a huge fan, and they opened an encore with a throwaway like either of those, particularly after the main set was full of new material, I'd probably be offended! (Of course, identifying those two songs as throwaways is my personal opinion and others may be delighted to hear those songs in an encore, which is why I almost didn't bother typing any of this. And I still maintain that 'Lightnin' made an incredible encore, and was always disappointed when they wussed out and did something like 'Kool Thing' instead. Oh, same with when they'd stick 'Kool Thing' in after 'Strawberry' to end the main sets in '02, what the fuck? You just play the most devastating 10 minute epic and choose not to rest on that?)
"I Love You Golden Blue" is another example -- here they had an absolutely perfect opener, and it always perplexed me anytime they wouldn't open with it on that tour.
// original folk blues ... . . . . . . . . . . . . . . . . . . . . . . . . . . .i'm the boy that can enjoy invisibility. . . . . . . . . . . . . . . . . . . . . . . . .