It solves the problem of using Node userland code to do heavy computation. But
a) serious developers know better than to do heavy computation inside of Node anyway, and
b) it comes at an enormous, almost unforgivable cost: working with C++ is a nightmare compared to working with Node when it comes to realtime web projects (I've been in those trenches)
What would be exponentially more useful in practice is a concise, well-documented way to hook (multithreaded) C++ code into Node's engine.
But that wouldn't be nearly as sexy as a promise of Node, but with native performance.