That is great! I recommend exactly this method! You have secured yourself against any and all thieves.
But, you should still make sure you have a backup, should you forget it. I mean, I also memorized my old phone number 20 years ago, but I do not remember it anymore. We all forget eventually, that's just natural.
What happens if you hit your head or get alzheimers or something? Nobody's "wetware" (aka brain) is safe. Having a backup is always a good option. You don't necessarily have to write it down or save it digitally. There are many ways of saving the seed so that only you would know where and how to retrieve it.
It's a unique way to secure your seed and very useful technique as long as you remember it correctly. One drawback which I see with this technique is in case someone loses his life then how his next of kin will access the Bitcoins? Memorising seed is good way to keep your seed safe from all sort of online hacking's but there are some limitations also associated with this strategy which we have to keep in mind if we are adopting this technique.
What I think is best way to secure your seed is to write it down on piece of paper and put it in safe custody with info to your next of kin.