I can see why people are asking - I run a forum about Dance Music and Dance Events, and a fair proportion of the forum community on my site take drugs or have taken drugs, and discuss their experiences regarding certain types publicly on the site. With the site now being 10 years old, many of these people have grown up and grown out of it, have settled down, got a job, family etc, and they wouldn't want this information to remain publicly visible, for their employers or family to find.
xF allows the deletion of user accounts already, but it doesn't delete or hide the linked content (threads and posts created by this user). The only alternative I can see is changing the spam cleaner settings to remove all of their posts, but that doesn't allow for easy restoration.
What I'd love to see in the next version of XenForo is the ability for an admin to "disable" a user account (hiding the account and all of it's posts from view), or for the admin to be able to fully delete the account and all linked posts they have made, like with vB.