udevd is used by everything, sometimes directly, other times indirectly. It's needed to create device nodes for everything from audio devices to webcams to storage devices to stuff like modems and terminals, graphics cards, etc. Most desktop environments call it directly for stuff like USB drive automounting. Some non-desktop systems like Android and OpenWRT with very constrained hardware have their own replacements for it, but that's not officially supported by the kernel developers and the kernel APIs it relies on aren't documented or guaranteed to remain stable - they want you to use udev (and soon that'll mean systemd too).

Basically, the only people who've managed to get away with not using udev are those with their own kernel fork and a completely customised stack on top. I'm not sure if any of them even use glibc or the GNU userland either.