I would guess that "historically" negative film was shot because they would have been printing RA-4 prints in their studio; as Cibachrome isn't for the high-volume kind of operation, although theoretically internegatives could've been used.
I would also bet that medium format was used more than 35mm.
That being said, as much as I hate to admit it, I bet less than 2% of wedding photographers aren't shooting digital. MF however would give you a legitimate advantage over digital/35mm, so perhaps there are still some people out there wielding Portra 400 in their Hasselblad.
All speculation of course...