I ran a successful consulting business many years ago (and hated it). One of the things you learn is that you live and die by the quality of the people you can deploy because that goes straight to your reputation. In practice, consulting businesses are very difficult to scale. While you may start out high-end with a great reputation because you really know what you are doing, it is very difficult to bring in additional people with your level of skill because there is almost no overhead to them working independently. As a result, "scale up" often involves diluting the quality of your overall team to the point where the rates you can charge decline as well. Consequently, you either have to run a very small boutique consulting shop that can maintain its standards and billing rate or run a large generic shop that makes up for the lower billing rate in volume. For people that care about the quality of their work, the former is more rewarding but the economics work out such that you will make about as much money by going direct as a highly-paid contractor to another company.
But I agree with your assessment of consulting firms and the scaling/quality issues therein. I also feel that consulting shops expenses tend to scale a lot closer to profits than some other businesses, so I stay out of it:)