Human memory is very unreliable. Even if you can memorize your passphrase today, your memory will become weaker as you get older and you could suddenly lose your memory if you suffer a head injury. Furthermore, if you use a passphrase that is simple enough for you to memorize, then it is probably simple enough to be cracked by brute force computation. The best practice is to create a long, random passphrase and make physical backup copies of it. Store the passphrase in multiple locations, separately from the PRO Series wallet cards.