I know someone who does this and puts random things in the envelope like dryer lint. While that's kinda funny imagining someone opening an envelope with dryer lint in it... I like to be courteous and issue a reply. I always take the credit card application enclosed, tear it off and write with a large Sharpie, "NO, THANK YOU!" on the application. They always ask me to respond by a certain date, so I try to honor that. :P
It always makes me laugh anyway... I feel good about giving the post office some revenue and sending so many annoying junk mail things back to the sender with my reply! I did this yesterday... and I sent out 10 NO THANK YOU replies. I felt good about that.
I don't know if this helps the post office or not... but it sure entertains me!