We had case sensivity at the beginning and it caused a lot of problems. Poeple complained that their email does not arrive because firstname.lastname@example.org
are 3 different mailboxes then. If you force lowercase, then postfix will put emails for all these 3 email addresses with different casing into the same mailbox. If the email address is case sensitive, then the emails will get delivered into 3 different mailboxes and if you dont create all of them as different pop3 accounts in your mail client, you will never see the emails for this accounts.
Even if RFC's might specify it otherwise, I have never seen a single mailserver that handles email addresses case sensitive.