Doug uses Pure Clean for the reasons he states.
Its a powder, I dont like mixing powders - time consuming etc, but I have Pure Clean, PB, Dry Blend, DFC210, Double Clean so you can see I am not against powder.
However, I prefer ready to use (RTU) fluids like HD, Nemesis, M-Power. I find then easy to dilute to a weaker mix, and as for cost - powder may be marginally cheaper, and in the big scheme of things a few pence more on a carpet is nothing.
And if I run out on the job I just quickly pour some more, add the right amount of water, and off and away we go again.