As an independent LSAT tutor, I've had a similar experience in hearing from my students that they did not find a prep course helpful. However, I agree that this may be due to a biased sample.
I suspect that courses can be helpful for those starting with lower LSAT scores (in the 140s) because most courses are focused on simply getting students past the 150 mark. (Those starting with higher scores and/or better aptitude may be bored in class, and those starting below the 140s may feel confused and rushed at the pace of the class.)
The question becomes whether courses are worth the time and money, as compared to self-studying with low-cost/free resources.