I thought that in a brainwallet you need to remember a phrase. The reason why I get the user to input a random sequence of characters is to simply immitate the mouse-movement entropy. I just tell the user to randomly press anything they want in the keyboard and of course, not to remember it.
You are asking a human to enter a sequence of characters on the keyboard. Even although you are asking them to enter something random, many won't. They'll use a name, a date, a reused password, a song lyric, something like that. Your own example even uses your username. Further, humans are not capable of being truly random. Even when you think you are being random, you aren't. Human chosen "entropy" is never random, and as such, is weak. There is a reason that no good piece of software uses human chosen strings to seed a wallet.
Alright, that's obviously wrong by my side. I could ask them to flip a coin and enter the result. However, what do professional wallets do to create entropy ? If they don't use SecureRandom, what do they do ?