[bawolff]

There are reasons to do this, just not because of expiry.

The main reason to have multiple certs is so if your host (and cert prov key) is compromised, you can quickly switch to a backup, without first having to sort out getting a new cert issued.

[miladyincontrol]

If getting a new cert issued is some sort of thing you need to sort out, as in a process that takes time, you've already missed the target.

[bawolff]

If you want a backup system its best if its self contained. When your site is down its easier to just run a single command to copy over a single file in your control instead of depending on an external service.