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One of the resons they won't delete posts is that it then starts to lose context, poster A and poster B have a discussion and are joined by a host of others c-z. If you take out poster B's comments there is a strong chance that the thread won't follow and even that poster A's position seems unreasonable.
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On other forums with the possibility to delete accounts I've seen that if someone deletes himself, the posts are not deleted, the only change is the username.
I think internally they delete the user completely with all the things directly attached to the account, but the postings remain and because the user doesn't exist anymore the username is replaced by a placeholder (i.e. "User123"). If the user is deleted that way the discussion doesn't lose any of its context.
I hope it's clear what I mean by all that.

Don't get me wrong, I'm just pointing out how this is handled on other forums.
Greets
DarkLink