Besides any modern negatives, there are historical ones that need preservation (my grandfather-in-law's WW2 negatives for example). We've been meaning to digitize them but time is always an issue.
It doesn't. Once film is fixed it's insensitive to further radiation, by design. Light is just radiation at a different wavelength from x-ray, if the fixing didn't work then the negative would fade out very quickly from ordinary light.
Yes, at some scale, if you put it in the jet of a black hole the radiation is going to destroy the image from physically blasting it off. That's not a reasonable concern to raise about film stability. Having your developed negatives x-rayed will not harm them, full stop.
The exception is that you shouldn't project slides for days and days on end, because the color couplers aren't as stable as grandpa's black and white negatives and do fade out eventually. Early color negative films had some problems too and would fade out over time regardless of radiation. But still, flying with a developed slide is not a problem, we are talking about the difference between a few mw of energy and sticking it right next to a 200W projector bulb.
Analogous to the "don't project slides for months on end" - I would not stick it on the radiation source and leave it there for months, but it's not an amount you need to worry about for a reasonable number of trips (let's say less than 100) through the scanner.
