I would think it's a combination the latitude offered by negative film, and the ease of commercial color negative printing. Printing transparencies well is not easy or cheap. For a wedding, you'd have needed a whole album of proof prints done good,fast, and cheap, which would be tough to do with slide film.

Slide/transparency was popular in other realms of pro photography for two reasons I know of. Because of the one-step processing (no printing), you got exactly what you shot and color/image quality wasn't compromised by the automated printing machines. Instead you looked at the images on a light table and saw it with your eyes when you have a good original. I wasn't involved in it, but apparently print shops were better at making good color reproduction for cmyk printing when using slides as their original.

If you are considering slide film just because it appears to have better grain, there are other options. The f5 is a nice camera and part of a nice system, but you could add a cheap used rolleiflex/hasselblad/pentax67/bronica/etc.. medium format camera for group photos and get the medium format quality for the group images. Having an unusual camera for the group photos might also help direct everyone's attention to the same place when posing. Unless you make group photos bigger than 8x10, I think the f5 will be more than adequate. I'd think only upscale clients would be ordering bigger than 8x10" group photos.