Fisrtly I think in the right situation, with the right carpet, then encap with a pad is acceptable.
It is just one of the tools we would use, out of a vast amount of machines available.
Does it clean as well as an all singing and dancing top of the range TM, again depends on what you are cleaning.
It is a great method for low profile commercial carpeting, is quick and cheap to use, which generally is what the client requires. I have used it on some domestics, but I will still fire up the TM for my 1st choice.
Does a pad spinning at 150-200rpm suck out as much from the base of the carpet, which generally is where a lot of soils are hidden, as a vacuum from a TM blower ? not in my eyes.
Yes you can put a pad over a freshly wet cleaned carpet and the pad will turn greyish, but personally I often use a pad after anyway, leaving it dry and deep cleaned.
I am not for or against encap, I just think we all need all of the tools of the trade to perform a wide scope of works
Andrew