Cat food is high protein which generally does not do dogs any favours...anything from weight gain, hyperactivity, skin problems, even to liver and kidney problems and these are all things I've heard of dogs suffering from when on higher protien diets so allowing a dog to eat too much cat food isn't a great idea. Even active working dogs can end up skittish and not perform properly on a high protien diet. I was reading where one guy who has a farm with dozens of working BCs and he trials them etc..mentioning how one dog was too pushy and forward and difficult to control until somebody mentioned his protein intake and when it was reduced, he improved loads and won his first trial the season after....and he was an active working Border collie. You don't get any more energy demanding than that in a dog.

The others have mentioned the Taurine issue anyway.