After carving your seed phrase on iron steel or writing it on paper and storing it in multiple locations, you can still decide to memorize your seed phrase, I don’t think it’s a bad idea, but if you don’t have other back up, and you decide to memorize your seed phrase as your only means to back up your seed phrase, then it’s really a bad idea, because with time you can forget something’s and you going to be losing your bitcoin, don’t think you so smart and you cant forget anything.
Using your brain and memory for remembering your wallet mnemonic seed (seed phrase) is only possible if you have only one or two wallets, if you have more than two wallets, this task becomes more impossible.
In addition, memory is short lasting, not long lasting so if you don't train your brain by recalling the seed phrase regularly like daily, you will forget it after some days, one week or several weeks. This raise a need of practicing with your brain very often, and I recommend doing this daily.
Even so, you will never know what will happen with your brain and memory and any accident might possibly cause memory loss, so let's say Bitcoin users should never rely on memory for wallet backup while I agree that if using your brain and memory as a supplementary wallet backup method is good. Just don't only use it as your only wallet backup method.
What about memorizing your backups?
Well, I can only think of one situation in which I'd ever consider that - if I'm fleeing from some calamity without much time to prepare and I'm worried about physical attackers accosting me during the journey or at border crossings. That is to say - I only think memorization is a good strategy for temporary emergency use. As a long term storage strategy it creates single points of failure:
You may forget the seed phrase.
You could be injured and unable to recall it.
You could die and your heirs would be unable to access it.
You could have seed phrase coerced out of you.