The gloves, not to mention the rest of the flight suit, are designed, first and foremost, to protect the pilot's hands and wrists from fire. All other concerns are strictly secondary. Like fine adjustments of knobs, pressing of switches, pulling cigarettes out of the package, and so on. Cutting the fingertips, or even the fingers, of them degrades that capability. How much, I can't say.
By the way, having been electronics most of my career, I can tell you that as analog, or resistive type rheostats (adjusting knobs) age, they can get twitchy. Harder to do fine adjustments with, jumpy, and so on. At least with bare finger tips, you have a finer touch on them.
I am quite certain that if they were discovered by an officious, non-pilot type, like a QA officer, or higher up in the Safety Department, there would be words had, or probably a Safety Stand-Down.
But you do what you gotta to get the job done. The guys that cut the finger tips, or again fingers, off are betting that either they won't be involved in a fire, or if they are, they won't be injured too badly with just that little bit of their glove missing. BUT, they can do the small, little, high dexterity jobs this permits. As needed.
Just make sure they get stuffed into a pocket before you come off the flight line after the flight. Leave no evidence

And remember, it's always better to ask forgiveness than permission. More effective, too.

Thanks very much, micro, for giving us your perspective on all this. It really is appreciated, not to mention informative.
Pat☺