Here is the motivation: it works and it has open peer-review. It is not exactly green OA, but it is a valid model.
https://peerj.com/pricing/ You pay a $99 for one article per year, to $299 for unlimited number of articles and time. But one has also to have a reviewing activity in order to keep these publishing plans privileges, one has to submit a review at least once per year (a review can even be a comment to an article). That’s a very clever mechanism which takes into account the human nature 🙂
In my opinion we, mathematicians are in dire need for something like this!
Speaking for myself, I am bored to wait for others to do what they suggested they will do.
(Only crickets noise until now, as a response to my questions here https://chorasimilarity.wordpress.com/2014/02/11/questions-about-epijournals-and-the-spnetwork/ )
Also, I believe that mathematicians form a rather big community today and they deserve better publication models than the ones they have. Free from ego battles and who’s got the biggest citation count.
We do have the arXiv, which is the oldest (true?) and greatest math and physics repository ever.
But it looks that after an early and very beneficial adoption of this invention of physicists, we are loosing the pace.
Moreover, if there is any reason to mention this, I also think that such a PeerJ-like publication vehicle will not harm, in the long term, the interests of the mathematical learned societies.
The same post is here too.