You don't want to mix systems - if you are switching between bodies and using various lenses you need to have stuff that works consistently and works together.

For weddings, your equipment needs to have a consistent feel - adjustments need to be simple and intuitive.

If I was mixing film and digital, it would be critical that I be able to use the same lenses on all bodies - that way if a body malfunctions the other body is available to immediately fill in.