I don't really understand the fuss about special guests. Not the endless speculation, at least. It's become another one of the strange modern Glastonbury phenomena. Don't get me wrong, when it happens (particularly the occasions where people least suspect it - Bruce Springsteen with McCartney, for example), it can be magical, but the mindless clamour amongst the camp I was in last year just before Elton came on was ridiculous. I lost count of the names that "someone they'd bumped into who works for the BBC" had heard. Britney, Dua, McCartney, Eminem, Harry Styles, etc. It's Elton John. Is that not enough? It turned out to be Brandon Flowers and a couple other lesser known artists. Probably didn't add anything at all to the overall performance.
Same situation the year before, Dr Dre and Eminem were supposedly dropping in with Kendrick. Turned out to be one of the most fantastically choreographed performances I've ever witnessed and not a guest in sight.
I have to agree with a few above. In hindsight, most guest spots that I've seen aren't that memorable.