1. From a product perspective, this is like going to a restaurant to get dinner but having to bring your own kitchen utensils, food and cooking your dinner yourself.
2. Anything running in a browser is inherently insecure - what's the guarantee that the site where you're pasting your key doesn't have some incredibly stupid security flaw and your key gets leaked?
3. Even if there are no vulnerabilities, you're still pasting your code in a random form somewhere on the web. All it takes is an ajax call or a websocket and someone, somewhere has your key.