Comment by account42

Email bouncing during rare downtimes is hardly that big of an issue - if its actually important the sender will retry, possibly with a different contact method. And for short downtimes most likely the sender's MTA will just automatically retry a bit later - email is designed to work with temporary failures.

There isn't some magic reliability that everyone needs which just so happens to fall into "not achievable with a single authoritative name server" and "guaranteed with two servers". I'm not saying you should never have more than one, just that isn't the registry's business to decide what kind of availability guarantees you need for your domain.