I would wish that a user himself had the possibility not only to change his preferences but also completely to delete his registration.
Here at MiniBB I discovered that I could not change my emailadress. Why? Because I had another registration -which I had forgotten - with the email adress that I wanted now. I had to change the email adress of this forgotten "account" to something not existing. I would prefere just totaly to delete it.
There could also be other reasons.
1. A user could suddently get paranoia about having his adress out there. 2. I as administrator don't like the possibility maybe one day to have lots of registred users who are non existing.
Not big problems, but.. :-)