You avoid maintenance tasks such as categorisation (changes over time continuously), watching out for vandalism, and reverting destructive edits. In addition, you are able to focus the documentation of currently used content (not that relevant for proposals, but in case of software lists or recommendations/comparisons).
Lastly, I guess some users like to clean up and condensing content (a similar process happens in the brain with our memories).
Going through the list set up by Polarbear, the process is straightforward: You check if it qualifies for deletion and otherwise you leave everything as it is.
Honestly, you could think of a read-only system for old proposals. Employing such a system would save some maintenance, because no one needs to watch the changes and you can basically keep everything (like with the mailing list archives). There would be quite a lot of “low quality content” though (proposals we currently delete for instance, although I am not saying everything I would delete is of a “low quality”, some things are just duplicates or discussions that never took place because no one was interested, proposals based on misunderstandings…).
Such a system would not be like a wiki, but rather like a database storing a log of all actions with no option to change it later on. User Ezekiel “stored” some of the proposals on a user sub page, but I dislike that because still everyone could change it, but it is more tricky as it is in a user space and it is not really related to this user.
Still, you would need to decide when a proposal is ready for archiving and this is not trivial either…
Edit: grammar, wording