Maybe today's critics would think of a '78-like performance as sloppy, but I find it hard to imagine '78 being sloppier than today's Stones. And critics today are often fans of dirty, punky music with a artistic and historical content -- exactly like the Stones of '78.
Anyway, I believe the tour of '78 is popular today because:
* It was back to basics in many ways. Stones classics, Berry numbers and great new material, done stripped down and punky. And no Billy Preston, just the classic '72 "swing band" with piano and a bit of brass.
* They played lots of brand new material which makes it fresh and original, still today. Even '75-76 felt more nostalgic and slightly incoherent in comparison.
* Ronnie Wood was an integrated part of the band and I think many fans see it as a definite high of his career. He filled Taylor's (and Richards'!) shoes well in '75-76, but in '78 he came to the fore even more and at the same time did that (in)famous weaving so well with Keith. His solo in Beast of Burden is enough to convince anyone of his qualities as a guitarist.
* With no official release (such as Live You Live or Still Life before and after), but many good bootlegs exposing this tour, I think many fans like to remind people about its greatness, so it gets all raved about on a board like this.
It's well deserved, in my opinion.