Which ironically, is a model with probably best performance and scaling characteristics. If you want high performance, you should avoid sharing mutable stuff as much as possible.
Because JS has no notion of immutability, sharing is done via copying - but what's worse, copying via full-blown serialization, not just in-memory blitting or COW. At least, that was the state of things when I last checked a year ago.
Besides, the problem with models like those of JS (today, anyway) is that they don't even let you read concurrently, even if there are no mutators.
Node.js (and PHP, for that matter) often don't get enough credit for their scalability in the large. Perhaps this is because the surrounding distributed-systems ecosystem hasn't grown up around them yet the way it has around, say, Erlang or Java or even C++, but architecturally they're pretty sound if you want to build distributed systems that scale across many computers.
We should use all the hardware resources available to us. If our programming models don't allow us to do that, then we should replace them with models that do.