Why do you clean a grill? Let the carbon build up. It's seasoning and food doesn't stick to it. Just preheat the grill to burn off any remains from the prior use. If there's really heavy buildup, use a straight edge scraper like a paint scraper or heavy spatula. Cleaning a grill down to shiny bare metal is a sure way to have all your food stick to it every time.
Grease and oils get rancid, particularly in hot weather, and rancid grease on the grates can make your food taste bad. Rancid grease on the bottom of the grates can vaporize and flavor the food, too.
The black crust on the grates is mostly carbon. It tastes bad. In addition carbon insulates the grates and can stick to food. So it is vital that your food go on clean grates.