I don't understand this choice not to add the End Membership. Everywhere on the internet you can join and exit whenever you want. The results are that people are not coming back and you get a list of inactive users the Administrator also have to deal with.
Well i had a wacko who felt trapped and misled due the fact she could not leave the forum. Wacko started to share it on social networks that she felt trapped and misled.
You get that anyway whether or not there's a self delete option. Some people disappear and don't return and aren't bothered about deleting accounts. You can see this on the likes of Twitter and Facebook which have delete options.
Put something in your Help about how to request account deletion. If you have XenForo 1.5 and you use the new user conversation facility, put the information in the welcome email/conversation that they can request account deletion at any time.
Personally I don't allow account deletion on my site (and I know many other sites don't either). I rename users and remove personal details from their profiles etc. Content (posts etc) aren't deleted as doing so would muck up discussions on the forums. I make it clear in my site policy about this.
We plainly list in our TOS that the accounts are part of the property of the forum.
There are many reasons for this, including those Martok listed.
People get angry, want to delete, then they want back.
Screws up the discussion threads and info.
Screws up member numbers (think advertisers who want eyeballs)
We have a group for "former members"- they get put into that group and their banner is "Former Member" so people know they are gone.