Agreed there are already a few frameworks that are working with ES7 in mind to ensure longevity. React & Angular are already playing with Object.observe (ES7) to avoid that nasty dirty checking.
Agreed, speed matters if it as you said comes with a more efficient implementation that is far more developer friendly. With ES6 and soon ES7, notably Object.observe, we may be getting the best of both worlds. However, in terms of speed for jsblocks I do not see the server-side rendering feature as a plus or really even a feature.
Server side rendering... What happens to jsblocks when it's faster to just render client-side in ALL cases? In some cases client-side rendering is already much faster. Under heavy server/network loads client-side is a more efficient & faster method.