I'm +0.5. Every time this bites me, I apply the same solution, so you're probably right that str.join should just do the work itself. And it's no doubt more performant that way, anyway.

And I've probably got some code that's just waiting for the current behavior to raise an error on me if passed the wrong inputs, even if I'd prefer it to succeed.

I should be +1, but I have a nagging "refuse to guess" feeling. But it doesn't seem like much of a guess: there's no other logical thing I could mean by this code. I'm unlikely to want it to raise an exception, or do any other conversion to a str.