Wow. Ive a few comments to make....
When I was a pioneer we were told plainly that hardly anyone came into the organization through the door to door activity. Rather most were people contacted by friends, family, and workmates. We had a cursory discussion on why the door to door then. Besides the fact that it still does bring in 'some' is the fact that it helps you build your faith.
If I had gone to college warning bells would have started sounding. Build your faith. This is another term for 'self-indocrination'. This in my opinion is one of the main reasons for the preaching work. When out in service you are surrounded by others like you. Surrounded by the publications, get a little bored waiting while someone is at the door, pull out a book and flip through it, or talk with the same people about the same things. What you are going to say at the door next, or some experience you had recently, or the fella who accepted a magazine five doors down and so on. If you do get someone at the door and get into any sort of conversation, you are reiterating the beliefs, often over and over and over again.
Say something enough times, no matter how untrue it is, and you will begin to believe it. Some kind of personal hyponosis perhaps, I would have to look into it more. But it is a common ploy of groups to alter our thinking. Remove our thinking from one thing to another. Specifically to their thing.
This again would be the reason for the emphasis on pioneering. Even if a particular family cannot pioneer, having someone in the family that is, lifts their status. I was not living with my father, was having absolutely nothing to do with him, but still the congregation thought it was so wonderful that his son was pioneering. This despite the fact that he had them convinced I was a thief stealing from him! A thief true, but at least I was pioneering, must have been their thought. (btw before another 'is hygh a troll' thread starts, no I was not stealing from him.)
A fella in this thread pointed out that it is more important to keep current members than it is to get new members. Well, I would state that is a fact, and thus the reason to continue the door to door.
Yet another reason, it is a hallmark of Jehovahs Witnesses. Some people know that the mormons do it also. But usually that question will be asked of the witnesses, 'arent you those people who come around with the bible door to door?' And from the perspective of the witnesses, since they have the concept that only door to door canvassing really counts as preaching, if they were to suddenly cease it, what then?
Also what would be taught at meetings? They have already derided the clergy for their fluff. They could not exactly go out and start youth groups, and support groups, and so on.
No, the door to door work is intrinsically tied to everything it is to be one of Jehovahs Witnesses. The other things might even eventually fall by the wayside, but this is their one defining quality.