> It's a nifty optimisation, but it's not about avoiding reflow, it's just another method of dirty checking, similar to that done in other frameworks like Angular or Ember.

There's also a lot of "implicit state" not captured in such a virtual DOM, like the user's current selection, the scroll position, form values, and so on. If these are blown away, it creates a really poor experience. The virtual DOM isn't just an optimization, it is actually a correctness fix.