The main reason I would think it's a good choice is because if you decide to get a CA certificate later, you just drop it in and you're done; no additional conf

The main reason I would think it's a good choice is because if you decide to get a CA certificate later, you just drop it in and you're done; no additional configuration required. If you don't have a CA certificate, you're probably not advertising your https:// https:// URLs anyway, so unless search engines are aggressively looking/prioritizing for https transport, it wouldn't seem to hurt anything to run a self-signed certificate there.