In response to my previous post someone pointed out that MX records have an obvious benefit of offering multiple servers at different priority levels.
I don't believe that this is a benefit for many machines on the modern Internet. Most systems that have secondary MX records implement them poorly, they have less SPAM checks on the secondary MX server, and it often doesn't even have a canonical user-list! This is a really serious problem, spammers apparently often target the secondary MX server (I don't have evidence for this but many people assert it to be true and it would obviously work so is likely to be true) and it's well known that spammers often guess account names (a quick scan of the logs of any mail server will prove this). These factors combine to make a secondary MX server without a canonical list of user-names a serious spam problem, it will receive mail and then bounce it to innocent third parties (the vast majority of spam has a forged sender nowadays).
If you have the ability to run a well configured secondary MX server with a canonical list of valid account names (which must be maintained independently of the master mail server for obvious reasons) then there is the issue of why you would want to do so. What problem does it solve? In the early days of the Internet mail storage machines were often end-nodes on the network, many hops away from the central well-connected machines. This meant that sometimes connections would time-out or the hop count (which was smaller then than it is now) would be exceeded. Having a well connected server being a secondary MX server was a significant advantage for a small mail server in those times (by todays servers almost all the mail servers of 1993 are small and the biggest servers of 1993 were medium sized by today's standards).
I just did a quick search for machines with secondary MX records (IE multiple MX records at different priorities). The only significant mail service with such configuration that I could find was gmail. Hotmail.com, ibm.com, microsoft.com, aol.com, and zonnet.nl all have multiple MX records at the same priority - this is a cluster of primary mail servers not a primary/secondary configuration. The evidence suggests that mail servers such as hotmail.com do not benefit from a secondary MX record, so I doubt that any other domain needs it either.
In response to a comment on my previous post, I have previously mentioned in mailing lists the issue of spammers attacking secondary MX servers, but I couldn't see it when reviewing my blog archives.
Above is day 10 of the beard.