Reading this tutorial I have to agree, that setting up a basic backup MX is indeed pretty easy. But problems are not too far away:

If you compare the configuration you are showing here with a typical modern anti-spam configuration almost everything is missing. This is what spammers like to see and that is the reason they like to send mail to lower priority MX servers, as they are more often than not not so well configured.

In my case using the herein described backup config in conjunction with my current primary MX setup this would lead to the following worst-case scenario:

My main MX server goes down. While our current mail server rejects well over 90% of the incoming mails (up to 50.000 per day over the last days) the backup MX would not. If the backup does not break under the heavy load our primary MX will, once it gets back online and the backup send it thousands of thousands of mails and none of them will get rejected and each and every mail will be virus and spam scanned.

It wouldn't take long until your primary mail server will shut down due to heavy load.

So, if you indeed intend to set up a backup mx, make sure it runs the same config in terms of spam rejection as your primary server. Otherwise you will notice an almost instant rise in spam mails that come through (relayed by your backup MX).