I totally agree that more noobs like the OP will lose their money which would be evitable if brainwallets were known only as a concept on well documented blogs and not easily accessible to noobs through sites like brainwallet.org. Still I can't see why it shouldn't be possible to memorize secure passwords. What is your estimate how long it would take for a sentence long, yet memorizable like this one to end up in a rainbowtable? With a mutation like every second word later? Without ever mentioning the sentence on the internet?
That's very hard to say because we don't know the kind of resources that might be invested into calculating rainbow tables. It depends a lot on things we can't know, like the cost of hardware and the future price of Bitcoins (that could be stolen). Also, over what time period? If someone extends their rainbow table every day and after 3 years is able to compromise your brainwallet, you're still going to be upset, even though it was secure for 3 years.
Also, to be super clear here when I say "brainwallet" I'm talking about the form where you turn a password like "stfu!" into a private key. It probably
is possible to memorize a randomly generated private key,
but it would certainly require some training in memory techniques that most people have never used, and assistance from software (e.g. to turn your private key into a series of words that you then convert into an imaginative story that you repeat to yourself every day).
So, the way Electrum does it can at least theoretically work, though I don't know if anyone has studied how memorizable the generated word lists really are, even with training. The way brainwallet.org does it cannot work because you just aren't going to randomly select words from your entire vocabulary, at best you'll come up with a long password that's just a grammatical sentence, and that significantly reduces the entropy because it'll be much more biased towards words like "the" and small sentence fragments that can reduce the search space.
I ever only made one Brainwallet for a friend with one Bitcoin. She is of the non-smartphone-and-better-non-computer type, so I promised her to give her the bitcoin to "this piece of paper". I made her think up five long words that are mutually unrelated. She wrote them down and I consider this a safe password until I hear of more serious brain wallets being breached than stfu! (five closely related symbols)
If they were really random words then that's probably fine, the average adult has a vocab size of around 20,000 words so that's 20,000^5 combinations which is certainly not as good as a real private key but is likely good enough for now (it's about 71 bits of security instead of 128). But people are very bad at thinking up truly random things, so I'd question whether they were really unrelated or not.
Regardless if you're going to write something down, then it's not really a brainwallet is it? It's then a paper wallet and you may as well let the computer choose the random words for you, it will do a much better job.