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Each of those options has been chosen by the mail list administrator.
As a general principal it's a good thing to know the system sending you mail
is genuine. Given the variety, there is no point in rejecting the email if
there is no certificate, but having a verified certificate could be used to
streamline any anti-spam processes such as not greylisting. I don't know if
postfix can do that yet, but it seems it would be a good thing.
I think that DNS attacks are rather rare. Though strong authentication
is useful for various kinds of application, it is much less important
for antispam (I doubt that spammers do DNS attacks to let their spam
through).