Each manager gets maybe 2 calls to the eye in the sky (cameras) per game. It'd stop a lot of controversy & crap decisions by officials. I don't think FIFA & UEFA are interested in listening to any proposals for it though which is a shame.
One crappy decision from a referee or linesman can cost a team tens of millions if they get relegated or lose a final by that decision(s). We're in the 21st century, if the technology exists why not use it?