The wedding band I play with always has used backing tracks. Like Pork Pie Guy said, there are verbal cues for the drummer so it helps keep things in check during the song. The back tracks were all put together by the bandleader, who is a recording engineer, (among other things) so the cues are all real cues recorded by him, sometimes as detailed as to quickly describe upcoming hits, when to switch to and from hats to ride, full snare to cross-stick, the kind of beat to play, when to go to a 4-beat snare pattern, (rather than just the backbeats) etc. There's always a count-in for all of that too.
As for the backing tracks themselves, sometimes it will be a keyboard part, backup vocals, reinforced keyboard horns, and that kind of thing.
Keep in mind, if there is an instrumet or voice on stage, it's also coming through the PA - it's not a lip sync. What this does for us is it allows us to have 1000+ tunes at our fingertips for whatever odd-ball requests might pop up, and it allows a bridal party to completely custom tailor the music for their event - chances are, if it's a good dance floor tune, either fast or slow, it's in our book. Everyone in the band reads music, so everything is charted for everyone (the bandleader is also an arranger) so even if it's a tune we haven't played in years (and this does happen) we can get through it well enough that the people in the crowd don't know that we're basically cold-reading the chart.
I'd rather it all just be well executed live parts, but for what we do and how we do it, backing tracks are a necessary evil.