What concerns do you think you're separating exactly, when you're dealing with presentation-tier markup and code that combine intimately to a single component?
I think a lot of the people worrying about this separation of concerns, and feel it's such an obvious problem that they don't even need to expand on it at all, are probably people who had to deal with hideous PHP (in the unenlightened days before Laravel, let's say) where views, business logic, and everything else get snarled up together in a horrible mess between some opening and closing PHP tags.
I had similar worries when I first saw JSX. Having used it quite extensively, though, it does feel like it's a sensible idea now, from a development perspective at least. That said, it does increasingly feel like the days where certain people could just write HTML and CSS, and other people could handle the trickier JavaScript, are long gone (especially with things like Radium for React).
