You aren't using your own domain because Google may forget to renew their domain, you are using your own domain so you are not relying on Google to make your content available. If you are afraid that your registrar may kick you out, you should also be afraid that Google may kick you out too - which actually has a higher chance to happen than losing your own domain (assuming you are paying for it).

Personally after losing my blog when Posterous shut down i decided i do not want to bother with 3rd party blog services anymore.