hook_user doesn't function as documented

When you alter the users table and want to update your field through hook_user it is blanked by the user_save function. Take a look at my birthday module for an example.

It comes down to the fact that if you set your custom field to NULL as described in the docs, the user_save function still uses it in its query. When you use unset() it will not be used by user_save().

So either the documentation should be updated that you should use unset in stead of NULL, or it should be fixed. The patch attached to this post fixes my issue.