As I said, you'll only receive tangible benefits from secondary MXes if
you routinely go down for >24 hours, since at that time some senders will
begin to bounce mail, instead of deferring it.
There are different ways one goes down. Last winter
it happened that I couldn't connect to some sites in
the USA. Traceroutes were getting lost somewhere in
seabone.net, if I remember well. It lasted nearly one
week. During that period, I was able to receive mail
whose replies were being deferred. I even received a
message informing me that my web site was down.
The magic comes if the secondary is on a differently
Yes, it doesn't happen routinely: it is for those times
when it happens.