@Joel... Why would you run your (Presumably equal) mail servers with different priorities? If they're truely equal, then you use the same priority in the MX records. Lower priority is only used when the higher doesn't respond. That's not load balancing. It's a backup MX... MX load balancing is performed via the RR DNS resolutions.
However the LTM WOULD be useful if you wanted an easy way to automatically provision extra MX servers at times of stress. And easily remove them (Quickly) when the load goes down... (Although you could doit with either MX records that don't respond till you spin them up, or even that RST because there's no SMTPD running... It would be cleaner with a LB. But I have yet to see something that would require that sort of complexity. Even with loads of millions of emails per hour.
H