A comment on LifeHacker web site gives a good hint on this:
A laptop was neither powerful, nor particularly portable. I always had to go back to a desktop for heavy graphics and video work. And it was never such a pleasure to lug it around from airport to airport, with a five or six hour battery, making necessary a second battery and charger. Ugh.
So I jumped on the opportunity to replace my laptop when the first iPad came out. I had to adjust my workflow a bit, but I found the best setup is an ultra portable tablet and a super powerful desktop. (source + article with more comments