I've considered that before, but it raises several questions:
 - Although bumps every 24 hours are allowed by the unofficial rules, should they be allowed? Bump posts are worthless on their own, and their existence is really a dirty hack. The modified bumping system on some boards is a better model, but doesn't fit all boards.
 - Although "reserved" posts are allowed by the unofficial rules, do they actually serve any real purpose? I tend to think that their only legitimate purpose is to allow for future expansion past the 64k limit (which is kind of lame to begin with), and 95% of the time they end up being useless in that respect.
 - Especially when several hours separates two posts, multiposting can sometimes be the natural flow of the conversation. For example, if some news has happened since your last post, it'd be natural to make a new post rather than editing the old one.

The purposes of the unofficial rule are:
 1. Prevent excessive bumping, which is unfair to other topics.
 2. Prevent people from monopolizing pages of topics, which is unfair to other posters.
 3. Prevent padding of postcounts and similar, which is basically spam.

Quite often, a person will multipost without having any of that abuse in mind. In these cases, it causes unnecessary friction when their posts are deleted; they feel as though they are being persecuted by mods, or like someone is reporting them for this kind of "parking violation" as a way of attacking them. But we can't look inside a user's mind, so it's difficult to determine "legit multiposting" from possible abuse, and it's easy to err on the side of just deleting it and telling them to consolidate their posts. Not ideal, though.

I've been thinking that a more ideal solution would be:
 - Get rid of the unofficial rule.
 - Prevent posts within 24 hours from bumping the topic, which addresses purpose #1.
 - Prevent posts from being made when the poster has made more than 3 of the topic's last 6 posts in the last 24 hours (or something like that), addressing purpose #2.
 - Only allow one post in a thread to count toward post-counts per 24 hours, and perhaps hide/limit signatures in the excess posts as well, addressing purpose #3.
 
I think that the first three would be fairly easy to implement, but the last is more difficult. Also, it'd be nice to get rid of the 64k limit, but that's also difficult to implement.

The status quo is not terrible, so the matter is low on my to-do list.