Doctors are not omniscient. The number of drugs and diseases and their interactions is beyond any person's knowing. Take a look at Life Extension magazine, where the citations on a single article frequently exceed 100.
Drug ads bring the attention of consumers and doctors to potentially beneficial products. And they annoy the rest of us.
What's a drug? If you have the wildflower foxglove in your garden is it a drug? Is it a drug when you pick the plant? Are the digitalis-containing leaves a drug? Or is it only a drug when you extract digitalis (digitalin, whatever) from the leaves? These are important questions, because they determine the legality of many innocent actions. The FDA has long attempted the overreach of restricting the production and availability of food and herbal extracts, claiming that those things are drugs and subject to the FDA's powers.