According to basic terms and conditions which are there by default in phpBB, MyBB, IPB and vBulletin, anything posted on your forums is solely your property. This means if a user posts something on your forum which belongs to them, they are also allowing you legal permissions to use it and redistribute it. That is there by law and unless you remove it from your T&C, will always protect you.
Interestingly, there was an investigation by one of the EU commissions into account deletion (note that account deletion is not the same as removing all the content posted by that account) and they took the view that account deletion may be a required facility upon request, and it's partly why Facebook had to introduce it (properly), because in FB's case, it was potentially a matter of privacy. Given how much personal matter could be posted on a forum, it certainly provides a sort of precedent.
As for members who want to be deleted - I no long longer allow it - but I WILL put their account in a 'closed' group so they can't rejoin using the same info. It also means I can tag their posts if I need to later on (YaBB's delete member posts doesn't have a 'delete all by poster function'.)
I never understood wanting to be deleted. If it's all that bad, why not simply just never come back?