So, if they're just dull replacing them won't do any good?
My understanding is that "dull" is usually the glass (okay it's not really glass) or reflector. If it's the glass there are polishing compounds that will restore clarity. Unfortunately, the work for about a month before you have to polish again. Some also include a coating that protects them somewhat. They last for several months but the coating peels off and really looks ugly (think paint peeling off on the side of a house).