According to http://www.beautifuldragons.com/Nutrition.html, butterworms have good protein and low fat content. (At least comparable to crickets and better than phoenix worms which have been approved as a staple feeder).
My beardie ignores phoenix worms as they are too small and hates crickets at the moment. I've been cycling him on superworms, silk worms, and hornworms until he gets over his hatred for crickets. I live in Canada so I am unable to get Dubias.
You could possibly use it as an added very expensive staple feeder, however butterworms if im correct have to go through radiation before entering anywhere else other than where they are from so they do not go into moth stage and damage local ecosystems if escaped or released. So thats probably why people dont approve of them as a staple feeder as they have to go through radiation and who knows what might happen if given too often.