I'm going to throw an assumption out there: very, very few people are doing this. Yes, you could - in theory - "dock" your phone/tablet and do productive things with it. But a really top-notch phone is going to cost you $600+ and a really low-end computer that can kick the shit out of it will cost $200. I think that anyone who can afford the monitor, keyboard, and high-end phone will probably not sweat the cheap cpu too much.
So in the end, while I'm sure there are people in the fringes doing productive things on their phones and tablets, for the vast majority they are toys. This is not meant to be a disparaging comment - I have a smartphone, I have tablets... but I don't do anything more productive on them than take short notes and check email. Mostly they are consumption devices.