I get asked for big files as well. Early in the summer, I made the jump to MF with P30+ and Mamiya.
With that said though, what I dont like about MF is the speed. the p30+ can shoot fast but the autofocus is a joke compared to canon. and I use autofocus 90% of the time.
I think it just depends on what you're shooting. If I'm shooting a portrait of someone I only have 15 minutes with, I'm reaching for the 1DsIII. If I'm shooting a portrait of someone I have 2 hours with, I'll use the p30+ .. At the end of the day, the p30 files looks better every time and will for the next couple years...
Another debate is though, does that difference translate into a 4 color CMYK press. IMO, I don't think so.
One thing that is very nice about the phase files is they dont require near as much post processesing... If Mamiya had a little bit better AF system (maybe 3 points, instead of one) I would never look back =)