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This has been a topic of discussion on another forum that I participate on. We don't have an XP system or a rep system, but it is generally accepted that there are some all time posts.
Recently, we had a discussion about asynchronous communication (which is of course the style of any online forum) which was the result of two or three really good posts that garnered no discussion. After a week and a half went by, the original poster followed up with a statement to the effect that she was unhappy that the posts hadn't generated the amount of discussion that could be expected of the topic.
It was noted that the beauty of the forum, and all online forums is that when people are too busy, they won't necessarily just drop the post, but make mental, physical, or digital note of the post (like the Personal Nodelet), and get back to it when they have a) more time or b) something to contirbute.
A month ago, I kept getting "random" XP, and while I can guess some of it, I know some more was probably related to the upvoting of old posts that provided insight, solution, or possibly another approach to a problem.
Lexicon commented to me that he gets to vote 6 days worth of votes over his 5 day workweek (see his location to figure out how he accomplishes it). I don't know that the timezone matters too much, because I notice that at the end of the workday, I still have a dozen votes to spare, usually.
So, while your points are valid in an Mtv generation world, I think that with experience (of using the monastery), old nodes get mined for information and get voted well after the fact. I know some nodes probably do suffer from the Mtv style voting (you mean that node exists??? But it's not on Newest Nodes!), it still can't explain the "random" XP that comes up in the XP nodelet every once in a while.
ALL HAIL BRAK!!!