Maybe I don't want my email sitting around in your MTA queue for your sysadmins to read.
Given the volumes of mail that pass through these kinds of things, that's not likely to be a problem. More likely to be a problem would be the fact that the mail might sit there for a week before it gets retried a second time. That takes careful system engineering for load, making sure to retry old messages often enough, etc....
I'm afraid the technology to rapidly sift through large volumes of information to search for specific areas of interest is widely available. It is totally reasonable to not want to send mail through your ISP's mail servers and perhaps directly to a trusted mail distributor over an encrypted link. Of course, you can easily use a port other than 25 for this purpose. The problem comes when the recipient tries to validate your origin address against your secure mail server. DS