The size of the supplemental index and the pages included in it change as the web grows and Google changes their crawling priorities. It is a moving target, but one that still gives you a clue to the current relative health of your site.

If none of your pages are supplemental then likely you have good information architecture, and can put up many more profitable pages for your given link equity. If some of your pages are supplemental that might be fine as long as those are pages that duplicate other content and/or are generally of lower importance. If many of your key pages are supplemental you may need to look at improving your internal site architecture and/or marketing your site to improve your link equity.

Comparing the size of your site and your supplemental ratio to similar sites in your industry may give you a good grasp on the upside potential of fixing common information architecture related issues on your site, what sites are wasting significant potential, and how much more competitive your marketplace may get if competitors fix their sites.