The users' city variable is fairly meaningingless for those that are teaching off line - all my students at least live, and half come from the same city.
But it is very useful. I have used it for allowing students to specify their student number or the time and day of the class that they are taking (when their are multiple similar classes using the same course page). This time I decided to allow students to provide their mobile phone email address. This is very handy because it means that I can look at the users page and send students a mail to their mobile phone immediately. I increased the size of the input field in the /user/edit.html. But alas the database item length is restricted to 20 characters. It would be nice if this were increased to 50 or 60 characters to all people to make use of this field.