Russ,
It's a fair question.
I had a Flexi 5 pile lifter which way out performs your standard commercial vacuum.
We did some tests on domestic carpets. Flexi 5 with a clean filter. Next to nothing in the Flexi filter, but after cleaning the carpet, the in-line filter to the TM (previously squeaky clean) has grit and junk in it which the Flexi had missed.
I think that poses the question, why waste time vacuuming a carpet with a tool that is a fraction of the power of what you are just about to clean it with? The contents of both filters IMO provides the answer.
Simon