Some organizations set up their MX records so there's an offsite backup MTA to receive mail (perhaps that should read "many organizations", I have no data). Is there still a justification for doing this?
In my simple view of the world, you simply don't need a backup MX. If your primary MX is unavailable, mail should still queue at the sending MTA for several days. The sending MTA should continue to retry periodically until your site is available again. In many ways, backup MX configurations are an anachronism -- a holdover from the days when connectivity was unreliable and some MTAs' queuing algorithms weren't great.
Backup MXs can cause problems if they don't do the same spam filtering that your primary MX does. This can cause backscatter.
If your primary MX is down for some time, a backup MX could also cause backscatter spam with "delayed" DSNs (delivery service notifications). On the other hand, not using a backup MX would usually allow the sending MTA to generate the DSN, which is a much better way to do it.
What do you think? Are there circumstances where a backup MX makes sense for you?