As long as you have enough entropy, I do agree brain wallets are perfectly fine. The reason why many people including myself advise against it is that in general people are too lazy to properly choose random words instead substituting them for words they believe to be random or ones from famous films/songs/poems et al.
It's not just that. Everyone can work on cracking a brainwallet from anywhere. To crack a wallet.dat file they have to have a copy of it.