If you're throwing weirdly shaped things inside code other people don't need to interface with, that's fine. Go nuts. You'll be creating a circumstance where any new person working with your code has to learn that you're doing something strange, but you won't really be impacting anybody else.

The fact that JavaScript allows non-Error-derived values to be thrown means that the first thing that any code that works with a thrown value has to do to be technically correct is runtime type inspection. You can't even assume the thrown value has a stack trace attached to it. So while technically the ship has already sailed, we add further complication to everyone's error handlers when we emit non-Error values via throw... Everyone's catch code has to grow support for handling whatever Byzantine type we've decided to toss out of our code today.