I have never let them into my house (my house is much to messy to be letting anyone in!), but have stood at the front door chatting for a bit. I am Catholic, and make sure that they know that about me upfront, I am not looking to be converted, but I do enjoy chatting with anyone and everyone. And there have been a couple of times when doorknockers have completely turned my day around from absolutely horrible to something much more bearable - so even if they perhaps thought they were wasting their time (which I doubt considering they keep on coming back) I don't believe they wasted their time, but rather done something of worth.

Mind you, I have turned away my share of doorknockers as well - generally the really pushy ones who happen to knock at the wrong time, but I try to do so nicely, if firmly.