[andrewingram]

Ultimately it depends on what your concerns are, and how you want to separate them.

I'm not just being silly either. Separation of technologies (e.g. HTML/CSS/JS) is just one of many ways of splitting things up. The component paradigm is essentially a fractal version of separating by feature. Ultimately it's an architectural choice and as such any best practices should be strictly scoped to the problems they're solving.

The reason why many of us in React-land are looking to bring everything related to a single component into one file, is maintainability and strict encapsulation. Some of this is to do with developer-experience (DX), i.e. adopt patterns that make our lives easier and ultimately minimise hard-to-fix bugs. But we're also trying to adopt patterns that benefit the user (i'll be the first to admit we're not there yet). To that end, a component architecture also makes it easier to let build tools automatically take care of the nasty problem of making sure users only download exactly what they need for what they've asked to do.