UTF-32 is easy to understand for educational purposes but it's probably a mistake to use it as a real string representation, and almost nobody does. Code units and code points are the same thing in UTF-32 but they're different in UTF-16 and UTF-8; you can teach someone UTF-32 before they understand the distinction. Obviously, UTF-24 isn't used because it isn't a standard encoding, and if you really wanted to save memory, you'd use UTF-8 instead which is even more compact yet.

As for UTF-16, today the only reason people choose UTF-16 for new projects is because it's the native internal encoding of the ICU library. If you're not using ICU, it's pretty hard to defend anything but UTF-8.