So instead of storing the cookies with your password and username on your own computer they're now stored on their computer. Same with your creditcard number, address, and all your other personal data.
Instead of protecting yourself you've just given someone you've never met everything they need to do all the things you wanted to be protected from in the first place...
Edit: Why add another step in the process of transmitting data to your bank? Normally only your bank can see the data, when you use their service they too can see it. Do you want that?
Sites like this play on people's paranoia about them being monitored (usually by the CIA or NSA, always popular as Big Bad Evil) to get them to spill their guts to some total stranger. Then they find themselves victims of identity theft or have their bankaccounts cleared out, charges made against their creditcards they never ordered, etc. etc. [ June 23, 2004: Message edited by: Jeroen Wenting ]
Well, the purpose of such a service, generally, is so that your boss can't see where you're surfing, or perhaps so that your browser history doesn't show the sites you visited.
But note that the address you give is an http: URL, so that communications between your browser and the Proxify site are unencrypted. That means your company can see all the communications on the wire, so they would be able to know where you are surfing anyway. And futhermore, if you type "http://www.javaranch.com" into the text box and hit enter, you and up looking at the Saloon with a bunch of adverts along the top of the window (most of them for porn!) and this URL showing in the address bar:
So in other words, JavaRanch will show up in both your browser history and your company's normal HTTP proxy logs. This is a severe problem with this particular implementation; I can certainly imagine a much more discreet "proxifier" than this one!
Now, finally, if you use Proxify to access a secure site, note that they have the opportunity to decrypt the communications before establishing a new https link with your browser; therefore although all the Internet traffic is encrypted, the good folks at Proxify.com would have access to all your banking, credit card, and other information.
So it's up to you whether you think that a company whose main interest seems to be selling ad space to pornographers should be trusted with all your sensitive data.