But today, with computers and intarwebs and such things, these records are free and pretty broadly available. So someone who, say, wants to letter bomb the residents and owners of every apartment in a tri-county area, can do it with virtually no effort or expense.
Not sure this is a very good argument. What's to stop someone from doing the same thing using a local phone book?
No man is an island if he's on at least one mailing list.