This format is more akin to RSS than to a programmatic rest API. The main goal is to be able to avoid the pitfalls of parsing Atom and RSS feeds. Both Brent Simmons and Manton Reece are quite active in making decentralized alternatives for self publishing for which RSS is the current backbone.

Parsing RSS and Atom feeds is a solved problem, no?

JSON Feed is a new solution for the problem already solved by RSS or Atom. It makes it easier to develop new publishers and consumers. It also tackles the main problems with these two formats, e.g.: no realtime subscriptions, mandatory titles which are a pain for microblogs, potential security problems with XML and so on.

Like somebody somewhere has written: If no one had ever reinvented the real wheel - our cars would be rolling around on big wooden logs