The intense heat created by this cleaning method can warp cooking grates, crack ceramic igniters, and cause metal fatigue in the burners. Worse, the captured gases can combust, gas lines and hoses can melt, and the temperature inside enclosed carts can exceed the safe temperatures for storage of propane tanks. Why should anyone take my word for it? You don't have to. The official position of Weber is:
Weber does not recommend this type of method at all. Covering the cooking surface could trap gas and become a fire hazard and blocking the airflow causes very high heat that can damage valves, burners and supply lines that may make the grill unsafe to use. We do recommend consumers always preheat their grill on high for 10 – 15 minutes. This will burn off all of the food particles and prepare the grill for grilling.
Char-Broil has this to say on the subject:
We agree, it’s simply a bad idea to clean any grill in this manner. It’s also particularly dangerous. Blocking off the grate can overheat the entire firebox, and force heat into areas not necessarily designed for excessive heat. The firebox could warp, the porcelain could overheat and bubble or burn off/drip off, the gas train (valves, manifold, hoses, tank) could overheat causing catastrophic failure, hot flue gases could be forced out through the various holes in the firebox, which could lead to damaged side shelves. Consumers should always follow the manufacturer’s recommended cleaning methods.
Several other manufacturers have similar comments on this method. Unfortunately, even some reputable media outlets still point to the foil incineration method (as it is sometimes called) as the best way to clean a dirty grill. The truth is, this can be dangerous and since manufacturers warn against it, they may void the warranty if a gas grill is damaged by this method.