If you want a quick throwaway wallet use a site like blockchan.info. Not only are your funds in your actual account(not consolidated into some giant wallet), but the private key is never revealed to the server. They could not take your coins even if they wanted to.
Fallacy, has been discussed before
hereTL;DR : Your bitcoins can easily get stolen if the server gets compromised, the owner threatened or if he goes rogue, and that's simply because javascript is dynamically served from the server, it can be modified at will by anyone accessing the server.