ETA: It's not about bothering people who are busy to tell them they are wrong. Some have suggested that it's perfectly acceptable to be rude to people who come door to door...but if the tables were turned and someone treated you like crap because you wanted to tell them about breastfeeding, or no circ, or vaccines, or [insert thing you sincerely believe in and care deeply about] you'd probably be offended. Like I said, I can't speak for how the JWs do it, but LDS missionaries give up 1.5-2 years of their life, on their own dime to volunteer full time (and it's not all going door to door, they spend a lot of time just doing service for people). They don't do it to annoy you, or to bash you, or to interrupt you're busy life. They do it to share something that is important to them and has made them happy. I think acting like they don't deserve to be treated with as much respect as anyone else because you happen to be in the middle of something when they come by is a mean attitude. Some posters seem to think missionaries deserve to be treated like dirt and I think that's ridiculous. No one deserves to be treated like dirt for simply asking if you'd like to talk to them about something very dear to their hearts. Like I said, if someone was nasty to you just for trying to talk to them about your beliefs/thoughts/opinions/whatever I imagine you'd be pretty ticked off.
I don't much care that the JWs or LDSs or whomever are doing this because THEY think it's the right thing to do. They can care for my soul all they want, but leave me the hell out of it. Their religious obligations to spread their beliefs don't trump my own rights to be left alone, and if they come to MY door, unasked for, I'm under no obligation at all to be polite. I think what they're doing is incredibly rude and obnoxious. Maybe if they showed any interest in discussing MY religion too, I'd think it less obnoxious, but as was already noted, they clearly refuse to take literature from those they try to foist their own literature on, have no interest in sitting and listening to MY beliefs, etc. Why on earth should they expect me to extend them courtesies they're unwilling to extend back to me, IN MY OWN HOUSE?
I never asked them to give up any years of their own lives - they chose to do that. I never asked them to share what makes them happy, and I don't care if THEY don't feel it's an interruption or intrusion in my life because it's their calling - once they come to MY house, unasked for, they lose the right to expect the polite responses I might have for people I voluntarily entered into discussions with.