Not as a primary use case, of course. But if somebody wants to buy exactly one cheap single-board computer, why should they be prevented from being able to watch "non-free" content on it (as long as the DRM required to play it is strictly optional and does not come preinstalled)?

Chromecast still requires another device to send the video to it. Using a media serving device like a pi can mean you avoid tying up another device and the whole solution winds up being just like any other media system attached to your TV. Remote control, and interface.

I don't think that's how Chromecast always works (though it can work that way).

Most of the time, your device(s) tell the Chromecast what to play, and the Chromecast plays that on its own. There is integration which tries to sync the UI on the "controlling" device to the playback of the Chromecast but this fails often enough for me that it's clear that (in my case) my phone isn't streaming to the Chromecast directly unless I'm screen mirroring.

When the cheese is higher quality and cheaper with scrip, then regular money isn't "objectively better". It's a tradeoff and suggesting someone use scrip to buy cheese is quite reasonable.

But in this case a Pi is literally worse than either of those things at doing the video playback use case for the same investment. It's not a case of the Pi being as good but doing other things, you compromise the use case.