At the end of the day it's horses for courses, I have a desktop, laptop and smart phone and they all fill a niche. For day-to-day work my desktop can't be beat. It was chosen for pure grunt. My laptop was chosen for longest battery life. My smart phone... well there's simply not much choice, mostly for making phone calls but mobile web browsing and email along with job time recording are the main things. I don't see any way that I could do "real" work on it.
@sordatos: since the size of a desktop computer is accepted after it converged decades ago, it will be similar even if it will have a positronic or quantum brain. Of course we will have small devices, but they will be only as good as their user interfaces. It is irrelevant to compare pure computing power and services to nowadays'. We will find new tasks, activities and... problems for the new capabilities.