It has more to do with chargebacks than verifying at the time of the sale, as far as I know. If I claim I don't recognize a transaction, the business can defend themselves a little if they have my signature recorded for that transaction.
I think it is for just this reason that MC is ending the signature requirement. They’re ain’t defendin’ shit if they think they’ll convince anyone that the 100 pixel-wide smear was done by me and not someone else.
People forget buying things. Our legal system has a higher bar for falsely disputing a signature.