No mailing list discussion really, it was decided long ago for Zend_Filter then there was the odd request from users (RFC822 is usually requested) which had the developers pointing out they could only accept a solution from someone who wrote an original version and signed a CLA.
But how do you prove that the email address belongs to the person?
That's a different question

. The Regex only validates syntax/format to RFC822, whether it exists or not is beyond its scope.
But once again: would it hurt not to validate at all?
Depends on the circumstances. If the email is being used as temporary identification (no/rare emailing of user) then it's probably useful to do so. If you have a long mailing list, then it's also useful to remove invalid addresses before starting a mass mail. Within reason, if the user must be sent a message, and they must validate from a link/address in that email message, then the point is probably moot - it's not strictly necessary then.
Another point is to provide user feedback - what if they submit an invalid email by accident?
Is it necessary? Depends. Is it useful? Definitely. Usage is optional.
When in doubt, check the source. But the regex seems very compact for an email validation regex.
So true

.
I might dig for this myself, but either it's a fantastic Regex or it's extra permissive. RFC822 is pretty complex. Once you get over the basics, there's a ton of detail. I won't rule it out as being a far more optimised regex until its proven either way.