Personally, I had to take classes like "Ancient Eastern Civilizations" while pursuing a degree in Audio Production. So while the term "useless" is a bit strong, I understand where they're coming from.
There were specific required classes that I didn't like, but they weren't useless.
Prerequisites, of course, prevent one from wasting the professor's time from unprepared students.
Sure, course is never 100% useless, but when in one course you can learn "10 things" and on this one you can learn "1 thing" and have to spend similar amount of time on them meanwhile you're working full time, studying on weekends and trying to learn other things for your job/career, then things are tricky and you gotta pick - you put effort into some classes that aren't really efficent or maybe some OSS side project?