Bad for most authors and publishers
Amazon is pretty predatory from the perspective of authors and publishers.
In this case, you get your book only in that subscription set if you are in the Kindle Direct Publishing (KDP) program and that you can only do if the book is not sold anywhere else, in other words: if Amazon as a bookseller has a monopoly on the published e-Book. Not as iBook, for instance. KDP is part of Amazon's attempt at building a (near-)monopoly. The new subscription service is another part of that attempt.
Some books of the Big 5 publishing houses like Harry Potter get in as special deals to make it look like you get access to everything important.
The way this market is going, it will become more and more a place where you can't make a living, except for a few big-sellers and the big publishing houses.
There is no good channel anymore for lower volume higher price goods: Selling your book as Kindle book via Amazon nets Amazon in most cases >65% of the revenues (65% plus a download fee depending on the size of the book). Only if the book is <$9.99, and only for a limited set of countries, Amazon will take 30% (plus the download fee). Not a good option for professional books which have low volumes and require higher prices and thus end up in the 'Amazon take >65% category'.
For fiction/non-fiction books that are (mostly) text, the MOBI/EPUB format works well. For more professional books with large and detailed diagrams and such, it is very poor. There is hardly any innovation on this front (even iBooks have serious limitations here)
The while eBook business works only for low price, high volume, low complexity manuscripts. If you don't fall in those categories, it's not always that positive.
Amazon has monopoly power. It shows. And it is never a good thing. Nice blog post about it from an author:
http://www.antipope.org/charlie/blog-static/2014/05/amazon-malignant-monopoly-or-j.html
Diversity in the press is at stake in the end. Here, the pure market ends up in a stifling monopoly and the politicians must do something to make sure market power does not destroy other fundamentals of society.