Here's a real world scenario for the suggestion in the thread title... At AWBM users can specify an image from the preferences settings to be displayed at the top of their profile page (example: http://awalkerbit.me/members/kevin.274/). To achieve it a new custom user profile field was created letting the user select which image to display. A template modification is then being used to display the image on the profile page. The template modification can be associated to an add-on but not the custom user field. To duplicate the feature on other XF installs either everything would have to be manually recreated or export an add-on with the template modification which still leaves the user field to manually recreate. Yes, it is relatively minor effort to recreate the custom user field but I'm just using the above as one example as to why being able to associate the custom field to an add-on would come in handy.