I should probably explain my rather atypical "use case" (as we say in the software business):
One of my guilty pleasures is high magnification macro photography (like 5x up to 40x). I started doing this when I got drafted to do the annual report and marketing photography for a former employer and found it an interesting technical and aesthetic challenge. I'm probably one of the few people on the planet who acquired a microscope shutter for the Hasselblad V series (
https://www.bhphotovideo.com/c/product/801284793-USE/hasselblad_microscope_shutter.html) and I have an assortment of Zeiss Luminars and Leitz Photars with which to pursue this particular vice (in conjunction with a bellows and way more extension tubes than Hasselblad has ever recommended stacking).
Even the slightest vibration or other movement in this sort of setup is disastrous - I've found the use of flash to be pretty much essential. TTL flash is really handy, since it eliminates the need to correct for bellows factors. I don't really have to worry about ND filters (hard to attach to a Luminar anyway), since the bellows compensation is usually in the vicinity of 6 to 10 stops, but the ability to both sync and use TTL flash would be really convenient. Since the advent of the third-party V-series adapters for the X1D, I can essentially use my existing setup to do this digitally, but I can't use flash easily. Being able to sync and use TTL would also make focus stacking easier.