Brain wallets are terrible. What happens if you forget the important details to it, even if you think you'll remember it? You might think you can remember the simplest thing, but it's a pretty big risk. The advantage of brain wallets is that you can combine them with commonly used algorithms to store your data in an uncommon fashion, like by storing parts of your brain wallet at relatives' houses.
Some of those were clearly not intended to be real stores of wealth, considering two passphrases were "the" and "wallet". That raises interesting questions about property rights if the original owners and the receivers were fully identifiable, with nearly all outcomes being entirely unenforceable.
But in a situation where the owner loses his memory or cannot remember his passphrase, what then happens to his BTC?
Nothing. If you lose access, it's no longer your Bitcoin and it will stay unspent until the end of time.
At least until serious quantum computing or new breaking algorithms come along, which may be decades, centuries, or millennia from now, if ever.
