I think if the couple wanted slides to project, it would be among the only reasons to do so. Most (as in every couple I've ever known) want prints and/or digital files, though. There is such a huge variety of shooting situations within a single wedding, and almost all of them require moving quickly. Some slop is involved in the shooting of every wedding, at some point or another. Negative film's extreme versatility and ability to survive less-than-perfect handling makes it the far superior tool in this case, IMO.
But technical issues aside, I imagine that the couple in this day and age that wants slides is a very rare thing. It does you no good to shoot them unless that is what they want.