Quoting James Harper (james.harper@bendigoit.com.au):
This "spam uses highest number MX" used to be a lot more common than it is now, to the point where you could exploit it by having a tertiary MX that always gave a "try again later" and the spambot would give up whilst having no impact on legitimate email.
I remember reading about this ploy a few years ago, and hoisted a mug in honour of whatever Right Bastardly sysadmin invented it. Appreciate the news that the spam wars have moved on from spammer-using-highest-MX days. I've been not paying much attention to the latest skirmishes.
Additionally, I built some logic into my spam filter where mail could be rejected (or the spam score increased) by the secondary if the primary was known to be up.
Another fine idea. For my own use case, the previously cited woes, fixable though they might be, served to make me more seriously weigh whether backup MXes were worth the bother, and judge whether a single-MX scheme could be practical and reliable as alternative. The answers turned out to be no and yes, respectively.