Rush restates the obvious. Most know, and you are right when you say he preaches to the choir, what he is oging to say or where he stands. But it brings the faithful together. We listen and we beleive the same thing. He states it in an elegant fashion. He prioritized. This the left can never do. He can do it and still be enetertaining so that people cant wait until the next broadcast. Most who 'listen' to air america, (where it is playing), go through the motions of listening. In other words it is on as a show of support for their cause, but they dont listen or beleive.
I love it that the Left despises Rush Limbaugh but beyond that I have outgrown (not smugly) him. The only time I listen to the radio is when I'm in the car and I've found that listening to C-SPAN Radio (DC area) is more enlightening. Failing that, there is always music.
Talk Radio is a very rough and competivive business and I believe guys like Rush and Hannity are looking over their shoulders at a number smart folks who are coming up. The "15 Minute" theory is real. Some just have a longer 15 minutes.
I think the case is that Rush isn't speaking to the choir so much as he is speaking to people who may be undecided on an issue, or have no particular opinion on a given subject.