I think many actually read them and a good % respond. Usually it is a message conveying interest and gratitude for the e-mail you sent them, but in a vague manner that certainly fails to give you a clear picture as to where you stand.
I guess the argument you present is why send them since it seems they have low impact if any?
I think most agree that for the most part these applicant to program communications yield low impact. The counter argument is that perceived low impact is better than no impact. It takes little time and effort for the applicant, and I can't imagine how it would ever produce a negative outcome, somehow moving you down the list. Therefore, extremely low risk with low reward and in rare cases, perhaps moderate reward. Question becomes, why wouldn't you?
Obviously, if a % of PD's find it simply annoying, and it actually could move you down the list, then this clearly changes the equation.