Using http://127.0.0.1:[bitcoinwebport]/whatever is a bit limiting, it requires that the user has the bitcoin client, and that the client is running.

Using bitcoin:?whatever however allows us to add a uri handler to the browser, so it launches the client with the information if it is not running, or opens a window if it is running.
In addition it allows websites to be registered as URI handlers, so people could register mybitcoin or other wallet service providing website to handle the request for them, without requiring the application. I find this a compelling argument to standardise on this style.