I disagree with that as a blanket statement. I have some such ads running for which I don't encounter a sales person or utilize a creative 3rd party.
This month, I have an ad running for one of my sites in each of 7 different trade publications. I'm no computer geek, yet at the end of each day, I know how many visits each different publication generated, how many of those visitors made a purchase, signed up for a newsletter, etc. And I don't have any sophisticated software to do this, just a few little things I did on my own.