A user emailed me to say he/she was surprised to receive a message on long hidden address - the message was forged to be from a trusted or exclusive sender for the address, and so went straight through. The user reasonably believes that someone who was less on guard might have taken the message as legit.
It does make sense to discard all messages to hidden addresses intuitively, but we're not doing it - trusted/exclusive senders (including forged ones) and remaining counts still work.
Does it make sense to quietly change the logic to discard all mail to hidden addresses? If this is a real problem, that seems like the best approach to me. The alternative is to add a new feature that has to be turned on in order to do it - this prevents surprises, but it seems like it introduces additional complexity and most of the existing users will never know about it.