"just pocket money" has a way of growing through lazyness and the sum of all pocket money is great in any case.
1) New bitcoin user decides to start off by purchasing a 4 or 5 figure sum of bitcoin (in USD terms).
2) User wants to be extra special careful, so they spend some time creating a brain wallet and move their funds there.
3) Weeks or months later, they start to worry about forgetting the key, so they import it into a desktop client.
4) They forget that a sizable fraction of their stash is held in a weak key since they never moved those funds to a new address.
5) Bitcoin appreciates an order of magnitude or two.
6) Brainwallet key is eventually cracked.