A user who was later banned, marmot, attempted to delete all of his posts manually (by which I mean, one by one), getting rid of several hundred and turning a lot of threads into confusing monologues in which others debate "message deleted."
Until then, all users had unlimited editing privileges on their own posts. After that, the admin switched so that users can only edit their posts for up to 24 hours, to prevent this kind of retroactive historical revision. I fully agree with the purpose of this policy. You can't participate here in a multi-person dialogue and then try to remove your part in it months or years later. That wrongs everyone else who participated.
One thing that frustrates me about it is that we no longer have the ability to correct mistakes or fix up sentences that are incomprehensible, perhaps add material harmlessly (say, to a set of links on a given subject, etc.). I hate it when I discover old posts with typos, confusing prose, etc. Of course, if you're doing this a year later it requires restraint so as not to abuse the privilege, i.e. make yourself look like you were winning a debate, etc. All judgement call.
On the whole, it's better that people can't wipe the slate clean.
We meet at the borders of our being, we dream something of each others reality. - Harvey of R.I.
To Justice my maker from on high did incline:
I am by virtue of its might divine,
The highest Wisdom and the first Love.TopSecret WallSt. Iraq