I'm sorry, but there is simply no truth to that.

MX records aren't just for when you're down, so there's obviously no correlation with their effectiveness and downtime. It's most effective for large envrionments with multiple high priority MX servers that can load balance.

I think that you're confusing DNS MX with how the lower-priority MX server handles the messages that it's queueing for the upstream server, assuming you can't get the lower priority servers to at least process non-local final delivery addresses.

Seriously, read RFC974 and the documentation for your mail software for setting the maximum queue time for upstream mail and you'll learn that there's never an excuse for a bounced email.