I'm not really sure I follow you. Yes, we can avoid duplication with proper coverage of what we do have, but how is that a reason not to mention the things that still need to be explained? You say that we "can better make LW a cumulative project by working on what we have done, rather than what we haven't done", but why should those things be mutually exclusive? People work on the things which happen to interest them - a list may inspire a person with otherwise no inspiration to work on something on the list. The net result is more articles written, not less.

You say that "high quality articles will rarely result from people reading a list of desired articles and deciding to work on them", but provide little reason for why this would be so. Indeed, beginners may feel intimidated by having to come up with their own subjects and would probably prefer having a ready list of example topics they could work on. And even if what you're saying is true, that high quality articles will rarely result from people reading the list isn't the same as saying they never result from reading such a list - even a list that's of limited use is better than having no list at all.