It seems the more Google talks about the supplemental index, you know the results in Google that are labeled as "supplemental," the more confused SEOs are, including myself. Here is an image of what a result in Google.com from the supplemental index might look at.
Typically, there results only appear for very obscure queries. Why? Because, a supplemental by definition, belongs towards the back of a novel and it just enhances any missing parts of a complete story. So first you would get results from the main index and then the supplemental index when all is equal.
But traveling through the forums recently, it appears that Google is trying to give off the impression that the supplemental index is nothing bad for your site. Whereas, all expert SEOs used to and some currently, call the supplemental index, the sign of death (in terms of your site ranking in Google).
A Search Engine Watch Forums thread is named "supplemental not bad according to matt cutts?" And you also have this huge 17 page thread for about two weeks ago that is still very active at WebmasterWorld linking to Adam Lasnik's Google Groups post named The skinny on the supplemental index.
1) Penalty? When your site has pages in our supplemental index, it does *not* indicate that your site has been penalized. In particular, we do not move a site's pages from our main to our supplemental index in response to any violations of our Webmaster Guidelines.
2) Freshness? You can expect to see a fresher supplemental index in the coming quarters. By the definition of "supplemental," however, I don't forsee it becoming as comprehensive or frequently updated as our main index.
3) Cure? Get more quality backlinks. This is a key way that our algorithms will view your pages as more valuable to retain in our main index.
Ok, first two are great Google-like responses, IMO. But to call something a "cure," kind of implies that there is a major issue with being in the supplemental index.
At PubCon, Matt Cutts and Vanessa Fox both said that the supplemental index is not a bad thing. But is it a good thing? Not bad for who?
I would love to hear a definitive pro and con on the supplemental index from Google.
Update: There is an other thread at WebmasterWorld named Supplemental Results: What exactly are they where Adam Lasnik of Google posted to yesterday where he said;
Pages are in the supplemental results because we still wanted to be able to show them to users, but the pages didn't have enough PageRank to make it into our main index (which is more extensive and updated with greater frequency).
Getting more *quality* backlinks is generally a good way to get more of your pages in the main index.
Still, there is a lot of confusion on this.