The main reason - Armory's master key generated by Armory's algorithm. If the algorithm will be compromised, or will be picked up initial conditions for key generating, then your coins will be at stake. In addition, I think it is not easy to find out (and remember) a phrase for existing key.
I just like idea that you may use your favorite phrase to restore wallet anywhere anytime. Just select strong passphrase. In addition you may create paper fragmented backup if you like.
1. You're underestimating the security of Armory's random number generator. The non-technical version is that computers and algorithms are actually pretty great at generating a limited number of securely random bits. For more details,
read this comment in the source code (it captures a lot of info about your mouse clicks, key presses, system files, and a screenshot of your desktop for extra entropy).
2. You can generate a phrase for any given key/number following grammatical structure pretty easily, even if it might not make
perfect semantic sense (but that's okay, it doesn't need to be sensical, just memorable).
3. You're overestimating the security of a person thinking up a "strong passphrase". This is what everyone in this thread has been trying to tell you. Bottom line: If your brain came up with it, it's bad! (not 100% of the time, but often enough that that's the advice I'll give you)
If you want to memorize your Armory key, what I'd recommend is making an algorithm that can convert between the easy base 16 format and a passphrase format. There should be a one-to-one-to-one correspondence between valid 128-bit keys, valid 128-bit base 16 encodings, and valid 128-bit passphrases.