I've seen this kind of statement multiple times on meta:
If a question has been vandalized, independently from who did it, it should be rolled back
I assume the same is valid for answers. But what about deleted answers? I've seen users post a wrong answer, delete it and replace its contents with some placeholder. Sometimes that's done during the grace period, so there's no way to even know what the answer was. In this case, they become totally useless, just taking space there.
Should I do something about those answers? Should I rollback when possible? Should I flag when rolling back is not possible? Or should I just ignore them?