And to each his own, that's the great thing about living in a country that tries to practice the principles of freedom. If you don't like Rush, then you don't have to listen to him.
Radio does have a certain amount of entertainment value to it but it is also a media that can convey useful information. One of the reasons I listen to Rush is because I like the entertainment value that he has to offer. I particularly like the environmental update theme song that has a blues band with a chainsaw doing the lead. I've got to get a copy of that to send to an old high school friend who since high school has moved to Oregon and adopted many environmental views. I'm sure she will not appreciate it.
The name of the game in radio, and or TV for that matter is numbers. So yes, to a certain degree Rush probably does preach to the choir as you say, in that his how he keeps his numbers up. But at the same time he has consistently exhibited a very well defined set of critical analysis skills that I find interesting. Although I might not always agree with his conclusions, he does have a logical and reasoned approached as to how he arrives at them. Not an emotional tirade that usually tends to be void of reason that many commentators have.
That is why I listen, to be both entertained and informed and find different ways to think about various issues.