Many advanced printing methods in the darkroom are benefitted by dimensionally-stable polyester film.
Some people do unsharp masking for black and white printing. I also do many kind of color printing controls which need film to stay in register once punched. Acetate films shrink over time, or in the short
term will change dimensionally due to humidity changes. Modern films, unless they are severely outdated when you use them, will not crack or shed the emulsion. That's not the issue. Some of these
procedures take several steps where either you need constant humidity or stable film. Most black and
white sheet film is polyester; not all color films were. And nearly all 120 film is on flimsy acetate....
sigh ... have to work with some of that tonite!