This is interesting, because it kind of bears out what I've seen in recent weeks. My main site has around 650 pages in the index at any given point. The theme of the site has heavily to do with events taking place around this time of year. As we got closer and closer to summer, more and more of my pages came out of supplemental, until now, when I've only got maybe 20 in supplemental (whereas three months ago, I had only 40 that were NOT supplemental) Search queries and traffic have naturally gone WAY up - I'm quite sure that search queries for a great number of those pages spiked in the past couple months - and I'm equally sure that most of those pages are rarely if ever searched for at any other time of year.
So that means my site will probably start falling back into supplemental, as time passes and the queries die down.