Call waiting isn't rude. It's how someone uses it that is usually rude. I have visual call waiting, so I can see who is on the other line. I don't answer if I don't know the name/number. I tell my current caller that I have another call and will be right back. I switch calls and say "Hi, I'm on the other line. Can I call you back in ___ minutes?" I then switch back to the original call and apologize and tell them, "I just want to note that call." Then I continue the original call.
Not to go too OT here, but what ever happened to getting a busy tone and trying your call later? Now someone needs to be told you're on the phone and to call back? Just another sign of the changing times.