I've been wondering why there so few apps that will adjust itself to the user needs? e.g. a user uses one button very often, so it will be bigger that the others. or a listbox item that is more used goes to the top of the list. I am aware that too radical changes won't provide good UX or someone else may want to use that app and struggle with it. Though, I do believe that properly balanced "dynamic" UI may be good.
Are there any other reasons?
EDIT: Say, my web browser after a month of using notice that I use a lot of tabs so it places the tabs above the address bar. On the other hand, if I'm using only 4 on average it could place the tabs next to the address bar (e.g. IE 9).
The argument to not to confuse a user is very good, but nowadays almost every app require some kind of authentication especially web apps, yet I don't see many dynamic websites except for language and ads.
Another reason against it it's a difficulty to write books/tutorials and help the users through helpdesk.