I believe that the staffers of many elected representatives typically count the number of constituents who were agitated enough even to send a form letter.

Think of it like a poll. Would you make the claim that politicians are willing to write off people who respond to polls just because they all answer the same question or the question isn't somehow the "right" one? Probably not.

Of course, it's not a poll because people self-select. But even still, if the report to the representative says "this week, we had 37 letters about gun control and 43,742 about surveillance", well, it's clear what the politician wants to say that he or she is doing something about when he or she next sees a reporter.