I suppose it's possible that the process just inverted. Rather than playing an extra song for a worthy audience they plan on playing it, but leave the option to bail if the crowd sucks. Or if somebody in the band is worn out. I've probably seen an encore dropped as often as I've seen an unscheduled encore played.
Me too; in fact, I've probably seen that MORE than the unplanned additional song. I'm sure there are others, but Marillion is the only big one I can think of off the top of my head; at Toad's in New Haven we were chanting "FREAKS" (well, the crowd was, I wasn't; not my favorite song) and H came out and said "What the fuck IS it with that song?? If someone can find me the lyrics, we'll give it a go" and he sang about half the song from a sheet of paper while Rothery played guitar. It was special (only time on that entire tour, so it wasn't a bit).
There's also the possibility that bands just won't play things they haven't rehearsed. I saw a Priest/Whitesnake tour where Coverdale dropped out right before they hit Dallas. We were excited because that meant Priest could play a longer set. Nope. We just got out an hour early. Presumably they just aren't capable of tacking on Green Manalishi or Electric Eye without some lead time (which is what soundchecks are for, really).
I have no idea if it is true or not, but the story goes, Cheap Trick was supposed to do a show at Red Rocks with.... I have no idea who, but the headliners had cameras set up to film the show. The headliner told Cheap Trick they could have the slot, and if they played a full set they could use the recording gear. So on a tour where they were playing an hour, tops, every night, they just showed up, played a headliner set, FILMED it, and later released it.
EDIT, so it is true, it was Frampton, and it was to be filmed by AXS TV. Cheap Trick actually played over 2 1/2 hours (27 songs; kind of long for them, even headlining; they're a 2 hour act, the setlist for the shows before and after were about 17, 18 songs, including the "Hello There"'s and "Goodnight Now"'s). Of course, AXS DID release it... but edited it down to an hour, so you're stuck with yet another version of "Surrender"!
The flip side is that I've seen artists do audience request night and play whatever they want for as long as they want. I saw Brubeck damn near dragged off the stage after the third encore. His band pretty much mutinied. No reason those guys couldn't have played til 0200 if the wanted.
I don't think I've ever really seen that, except in bars. There was a guy here, name is Gary Gidman (he just passed away, RIP) who used to walk in to this bar around me with his guitar and a three ring binder (that was honest to god, three inches thick) and sit down and just take requests. He'd play probably three or four hour-long sets and every song was a request of some sort.
I do sort of not like the faux spontaneity, though. I saw Night Ranger - who I love - at a fair here in Springfield, MA, and before I went I checked the setlists just to see, and they played the same four covers every show of the tour ("Crazy Train", "School's Out", couple others) and yet Jack Blades introduced them as if this was the only time ever, and we were REALLY taxing Brad Gillis' memory to remember playing "Crazy Train" 30 years ago in the two-month gig he had with Ozzy... and we were REALLY putting Kerry Kelly (or Keri Kelli or Kelli Kerry, whatever the f--- his name is, second guitar player, who played with Alice Cooper) on the spot as the new guy by "making" him play "School's Out" (a song he's probably played live 100 times with Alice). I don't know; kind of rubbed me the wrong way.