I think it already happened. There was a link where someone lost 5BTC in brainwallet. Better to use deterministic wallet with lots of entropy, and encrypt with a good passphrase.
Estimate of how often it happens depends on many things:
1. How long is the passphrase
2. How many people use brainwallets
3. How good are people at using unique passphrases and a mixutre of characters, numbers, symbols etc.
Estimate of how often it happens depends on many things:
1. How long is the passphrase
2. How many people use brainwallets
3. How good are people at using unique passphrases and a mixutre of characters, numbers, symbols etc.
it has happened.