I use my personal computer professionally (independent software developer) and haven't used a desktop except for gaming in ages.
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The average computer user (who doesn't write software for a living) should be more than comfortable using a laptop as their only device.
At my previous software gig, everyone migrated to using Apple laptops plugged into large external monitors, with external mice and sometimes keyboards. Lots of people worked some days from home, so nothing beat having the one machine with everything 100% up to date, to work from wherever. I got my MacBook Pro in anticipation for working while traveling, but even when I got back into the office permanently, I still preferred it over a desktop. Often you'd see people carrying their laptops to meetings or over to someone else's desk to show something.
At my current software job, everyone was using desktop PCs when I started, and they walk over or remote desktop over. Recently, a chunk have switched to laptops, since they can better be used for demos or brought on-site.
At home, I wouldn't want a desktop. We have two main work areas at home, just so it's not the same setting all the time, so it's nice to bounce between those and whichever coffee shop we're going to that night. I use my laptop to watch movies on the coffee table while I'm on the couch. I only have a 13" screen, but we never bothered buying a TV since a TV across the room wouldn't actually appear any larger from the couch.
Now I use an iPad for when I'm out at a coffee shop and I'm only reading or using iTunes U, for the better battery life than my current MacBook Air, easier fit in my bag, and less worry carrying something that costs half as much.