At its core, Bitcoin is all about granting users full financial control at any given time. For some reason, a lot of these users will store funds in an online wallet or exchange, both of which are fully controlled by a third party. If the company providing this service would shut down all of a sudden, there is no guarantee users will ever see their money again.
http://themerkle.com/3-common-ways-bitcoin-users-lose-their-funds/I would add ponzi schemes to the lot.
Some people even purchase bitcoins just to participate in ponzis.
Maybe also gambling? Bitcoins mean that anyone can gamble, even 5 year old kids! And how easy is it to earn a few mBTC these days.
Plus, there are some idiots who think there is a way to bypass the house edge and manage to get profit in the long run (martingalers, *koff koff*).
