XControls should be used when you have significant functionality that follows a control around. The thermometer is one case. Another example would be a units control with a custom option. Normally, the control is a simple string selector. When custom is selected, another control becomes visible to allow the user to enter a custom string. Another example is filtering or modifying inputs. If you have a string input, but you want to disallow certain sequences or modify certain sequences, encapsulating in a XControl makes sense.
XControls only make sense if you are creating something you will use multiple times. It is a way to encapsulate functionality and create reusable components.
