We are building Postmatic 2 to leverage the REST api (for delivering comments from email), but it degrades to use admin-ajax if REST fails or is unavailable.

It's a hoop to jump through, but by taking a progressive approach we get plenty of flexibility and immediate gains. I look forward to the day that the install base for REST is strong enough that we can use it 100% of the time.

Epoch 2 (for displaying and publishing comments from the web) is built completely on, and will require, REST. There is no looking back when it comes to loading comments via REST and displaying them with Angular. It's fantastic.