And JavaScript Harmony

Because of their switch to use a year instead of a version number, do they plan on doing these ES enhancements more often?

Yes. Because of how rapidly Javascript is moving, they want it to be more modular in its releases instead of waiting multiple years in between.

I think that's the plan. Whatever the committee can agree on and ratify by December 2016 will be dubbed "ES 2016" and so on.

The plan is to keep releasing them mid-year.

Yes, though as I understand, none of the later ones should be as big as ES6/2015. You can't accumulate that many changes in a single year.

It's a bit like WHAT WG's change of HTML5 to HTML (the "living standard"), but less drastic.