Due to the way PHPBB handles deleted accounts and their messages, I too voted "no".

The integrity of the discussions should be kept intact.

Perhaps a sticky can be put up to step through how a user can cancel all subscriptions and wipe out the information on the User CP so that the account becomes, for all intents and purposes, deleted.

The only additional step I might suggest is for the "subtitle" of the user, i.e. Forum Regular, Administrator, to be changed to "Dormant" or some such word to reflect the status that this user is no longer participating in the forum.

This can be done by an Administrator pretty easily on request and could prevent people from replying to an old thread or not-so-old thread and wondering why someone is being an **** and not responding.