Between uses of my Weber grill I bring the grate into the kitchen, soak it in hot sudsy water and scrub with a wire brush.
I've been to several friends' houses for cookouts in the past month. Every one of them had a heavy buildup of crud on the grates. One of them made a cursory swipe at the gook with the spatula before cooking, the others just plopped the meat on top of the black layer.
So those of you who don't clean your grate of weeks (months? years?) old crud, why not?