Relevant TL thread: http://chat.stackexchange.com/transcript/message/9390241#9390241
Related dupe: Delete softly when using the big stick. You may want to read that first, Andrew's given a much more thorough reasoning for why the feature is necessary.
Currently, there are two broad situations leading to account deletion:
Voluntary: The user wishes to delete their account, and uses the delete button or requests moderators to do the deletion.
Involuntary: This is when a moderator deletes/destroys an account. This is usually used for spammers or sockpuppets.
When an account is deleted/destroyed, the account page becomes a 404, and the usercard shows a greyed out avatar/username, with the username in userXXXX format:
All user data is removed (from a moderator point of view -- the system may still keep a few details)
I completely understand why the data should be wiped in the case of a voluntary deletion -- if I want to leave a website, I expect to take my information with me. It's a privacy concern if a site doesn't do a reasonably good job of wiping data of a user who voluntarily quits. SE seems to do that well -- I don't particularly mind if part of the data is buried in the database and accessible to devs -- as long as most of the data is wiped and users/mods can't see anything I'm fine.
However, with an "involuntary" deletion, mods lose access to potentially useful data. Links to posts, IPs, et cetera.
In the case of a spammer it means that old spam posts are harder to dig up if we want to figure out what to blacklist. IP data can let us catch new spammer accounts easily.
In the case of a sockpuppet, IP and activity data can be used to identify other socks -- and that is destroyed. Especially in the case of puppets created to circumvent a low-quality ban -- IP xreffing is a good way to find more low quality posts.
Here, there aren't any privacy concerns as the user didn't ask for account deletion.
So, for such cases, could mods get a "soft delete" tool? Here, the userpage becomes a 404 for everyone except mods. The greyed out usercards now link to the profile (not too important, it's easy to get the profile from the id with some URL manipulation).
All the tools related to IP/activity should also work for these users -- for example, if I run an IP xref on another user, this user should show up (possibly with a purple background) in the xref.
This way, when investigating new accounts for sockpuppetry or when battling spam, we get easy access to some useful data about the IP/etc.
Update: Now that mods are being encouraged to use deletions instead of >1y suspensions., this is needed even more.