What keeps me from playing as an omniward?

I just noticed the "spectate" button which allows me to spectate a friends game. I'm currently watching and wondering. What keeps me from logging into skype, spectating a friends match and being his spotter? Is there a delay? does it not work on ranked games? what are the rules for this function to prevent abuse?

edit: The answer to my own question is that there is a 3 minute delay. This makes sense. Thank you everybody.