I figure if somebody is going to make heavy mods they should learn how to start a child theme, but sometimes a designer just wants to tweak style.css and not be bothered with setting up functions.php, making a screenshot, etc. With churchthemes.com our approach is to include download links for dynamically generated child themes in our child theming starter guide: http://churchthemes.com/guides/developer/child-theming/

That way they can just modify style.css and upload or do more sophisticated things (some examples included with the themes). Very handy to simply refer customers to the starter guide when they want something extra.