This is the very basics of how the business world works, your counterparty risk of storing your bitcoins on those anonymous websites is the same as you storing your money in some company in africa...
And i cant believe someone brought up mtgox, did you guys forget what happened to that site? The hacking, password dump, and the complete bs explanation they provided. The website is made by a bunch of guys running some scripts in their basement. They got lucky last time and didnt lose everything. I guarantee you if the loss was large, the site would be gone. Mtgox further reinforces what I am saying you are trusting your bitcoin to anonymous websites that 1) will purposely try to scam you then run or 2) poorly built, unsecure and easily compromised.
Why do you want to expose yourself to those risk when you could just keep your own bitcoins. Is this really such a foreign concept to understand? And everytime one of those sites get hacked with all the bitcoins you guys store on there, it shocks the market and make bitcoin less popular when they get dumped. By contrast, if everyone kept their own bitcoins then if the website goes down it will be a non-event, as the loss will be minimal and it will not create market volatility.
Cant believe I actually have to sit here and explain why storing your money on an anonymous unprotected website is a bad idea. Is there a triple /facepalm photo somewhere?
Only those who take great risks will find great success. Live a little! If keeping a few hundred BTC on MtGox for immediate trading and profiteering means there's a slight chance that I could lose them all to a hacker, then I'm willing to take that risk. FOR BITCOIN!!