Each article is published along with a note from the authors who say why their work is important (even if it was rejected in the traditional peer review process). The first open letter is a pretty good instance of this in action:
It is not in the literature, and does not follow in any direct or simple way from existing results. In other words, it is, as far as we know, new.The paper that I had the hardest time getting published (this one) was also new in that it didn't follow from existing results. So, I know what these authors are talking about. Now, the paper that I had such a hard time getting accepted is widely cited.
Rejecta Mathematica figures to be a very prominent location, so these folks will probably have the last laugh.