It's funny because some people think of healthcare marketing in the same way as they think of marketing major brand name consumer products like Coca-Cola or McDonald's. Both of those company brand names are so ubiquitous and known all over the globe that if either of them stopped advertising, would there really be a significant drop in sales? After seeing a McDonald's commercial, do people really say, "We should eat there next time." In the same way, most people don't actively think to purchase medical services when they see a healthcare commercial. In fact, most people don't proactively seek out healthcare purchases or providers until they actually need them.
To my mind, healthcare advertising exists in two different realms, the macro and the micro. In the microeconomic sense, health care advertising consists of your local doctors, specialists, and hospitals. Most small town people really only have one local hospital and a limited number of local heathcare providers, so advertising on this scale probably doesn't have much of an impact. People in urban areas with access to numerous providers and hospitals do have choices and the advertising here probably does make a difference to get people to choose specific heathcare providers. That is why you will see urban hospitals advertise much more than small town hospitals. It's simply the size of the market and the choices available.
On the macroeconomic level, this is similar to the Coke and McDonald's advertising. It is not so much about getting people directly in the door, it is about building up a brand name thereby indirectly getting people in the door. Since hospitals and providers tend to work and deal more on the local level, this mainly applies to nationally branded healthcare like prescription drugs. In fact, drug company advertising is probably the single biggest slice of the healthcare advertising pie. Since the same drugs are available all over the globe, it is important to get people to want your specific drug since they can choose from most anything available (with their doctor, of course). For drug company advertising, its about both building up a brand name to compete against future competition from generics and also to directly get people to seek out a specific drug. In fact, the commercials all say the same phrase, "Ask you doctor about _____."
To summarize, on the local level, healthcare advertising has much less of an impact since healthcare at this level tends to be local anyway. The ambulance is going to take you to the local hospital whether you ask for it by name or not! But on the national level, it takes on more significance. Probably the biggest sector to take a serious hit if all healthcare advertising were to stop would be the pharmaceutical companies. They would almost certainly see a serious impact on sales.