It's not the case for any normal standard portable format (video, audio, etc.). They can be accessed on any platform. DRM prevents that limiting your options where and how to use it. If you take tapes analogy, you can play your tape in any player. Imagine a tape which is only playable in Sony player or whatever. It's clearly a defective product in my view.

> I don't think backups are relevant to DRM-laden streaming video, given that it's someone else's data that you happen to be observing.

That's exactly the point. It means usability is reduced if anything goes wrong with the service or you can't connect to it or whatever other similar reason.

> I'm not sure what you mean by a security and privacy threat.

Since DRM is based on distrusting the user (assuming all of them are potential criminals by default), symmetrical treatment for it from the user's side should be "always potential malware by default". Here is a good example of this: https://en.wikipedia.org/wiki/Sony_rootkit

Since DRM is always a black box / closed code, it adds to the reasons to never trust it.