A single exposure to a spot sells nothing. Which is why they buy reach and frequency. Because over time, the mindless listening gets the message in your brain---it's similar to name recognition for candidates.
I have always been of the opinion that an advertiser selling something needs to contact that specific individual(s) who are already thinking about buying and in that regard I agree with "a single exposure sells nothing". It is easy to miss that one person(s). However, the morning shows in my town run endless commercials for cars - most spots identical to the last per brand. Unless I am already thinking about buying a car those ads are wasted but not only that - they are irritating as well and can cause me to hit the change channel button. Just as the same spot aired endlessly for a political candidate is essentially wasted after the first 2 or 3 airings. Either I have already jumped on the bandwagon or have decided not to vote for him/her and listening to their blurb again and again just wastes my time and pisses me off.
And it isn't only spots that irritate, it is also content. During the past few weeks endless stories have been aired about the royal wedding in the UK. This morning one of my local stations began spewing about the anticipated color of the Queen's hat. ENOUGH! They are off my "must see" list until they quit polluting the airwaves with repetitious nonsense.
My point is that by irritating your viewer/listener you may cause them to change the channel and by doing so help them begin listening to a preferred service. I no longer listen to radio in the car at all because they drove me away with the above practices. I found I don't miss them a bit.