No one will ever see your Flavorizer Bars but you. Since they only vaporize drippings, how clean do they have to be. From an appearance standpoint, this should not be a big worry. After your first use on a new grill they are going to look pretty bad but again, who is going to see them?
Honestly, from a food safety standpoint, can they be a problem? Probably not when you consider how hot they get every time you light up the grill. They glow red and vaporize any juices that touch them. If they were a food safety hazard, I’m sure Weber would be issuing warnings.
However, Weber touts these Flavorizer Bars as a key to getting good flavor from a gas grill. By vaporizing the drippings you get the equivalent of meat juice smoke applied to the foods you are cooking. That makes sense to me! After all, when you put fragrant wood chips in your little smoker box it does actually add to the flavor of whatever is on the grill so why waste the flavorful meat juices? But it you don’t brush down the Flavorizer Bars when you brush the grill grates and if the grates are rusty and coated with many layers of charcoal-like soot, how much flavor can they add? Good maintenance is a prerequisite to good grilling.
This item also protects your burners. Mine are still going strong after 10 years so I know they work! It just makes sense to me that if you are replacing your Grill Grates you also should replace your Flavorizer Bars.