I only purchased a back after running a budget for a series of self promo/editorial shoots where costs were coming out of my pocket. Film, messengers, processing, contacts, etc would all cost about $8k US. I figured I could buy a back and sell the 1ds2 and come out even in a year or so, but I keep shooting film alongside the P30+. My commercial/editorial work is still 50% film.
I scan for placement only on a Microtek 120tf. I scan 4x5 on an Epson 700. They work fine, a little too much color aliasing on negatives, but they both work very well. These scans are nice, make nice prints, but are really just for giving the retouchers a guide to what I want. Lamount, Pixel Perfect, Box etc do drum scanning/retouching for press. One thing they taught me is never to scan negative film over 2400 dpi. You are just scanning film grain at that level.