yeah certain widgets lend themselves to easy use in html content and other widgets are more useful in custom development but not in the html editor. It doesn't really make sense to put a datepicker or progress bar into html content since there is no server side code reacting to them, but we use Datepicker in the blog and other places and we use the progress bar to monitor progress when sending the newsletter, and any jqueryui widget could be used in custom feature development, but of course you have to wire them up yourself from code to make them do what you want.
Best,
Joe