It conveys no useful information to the user. It'd be like prefixing all emails with "email:".
Wouldn't it convey that you are not on an https site?
but doesn't it break copy and pasting urls?
When you select (copy) the URL it silently adds 'http:// to it. You will see only when you paste it. I'm not sure I really like the concept of silently putting stuff in my selection buffer. I'm going to disable this.