Comment by TOGoS

Eh, if you're doing RPC where the whole request/response are already in another layer on top of HTTP, then sure, 200 everything.

But to me, "REST" means "use the HTTP verbs to talk about resources". The whole point is that for resource-oriented APIs, you don't need another layer. In which case serving 404s for things that don't exist, or 409s when you try to put things into a weird state makes perfect sense.