I want this option a lot. How? For example, moz got deleted in a SE site with a lot of excellent answers but user:moz returns /dev/null.
If a user is deleted, then while you can see his name, you are unable to search for his posts. Advanced searches are done based on a user number, so it would be interesting to know if you knew that number if the search would still work. If so, then maybe, they could have the number as mouseover on the username, so that you could search that way.
If a user requests to be deleted, they will sometimes remove his name completely, and replace it with