A good backup, e.g. Time machine, lets you restore feom before the corruption. A backup on an encrypted disk risks the entire backup volume being corrupted and unusable in one go, thus making it a very brittle backup.
I'm assuming that a corrupted encrypted file is totally unusable here, and now that I think more about it I'm not sure - encryption with chained blocks would mean errors have a larger affect than just at the error site, wouldn't it?
