The main reason to still use a keyboard and therefore a PC or laptop is for typing. I friggin HATE to thumb-type and tap on a pop-up on-screen keyboard. it takes at least 4 times as long as good old-fashioned typing. Of course you can TALK to most of the little devices, but for writing forum posts and such it will auto-correct and mis-guess to the point of making you look insane. No thanks!
Plus, a good old-fashioned desktop with a mid-to-high graphics card will cost around $500 with a 27" monitor. Laptops with decent graphics cost at least $1000 and the screens are now the limiting factor in usability.
If you travel a lot, then a laptop makes sense. I don't. I actually work from home, so I go with the most usable available device, which also happens to have the most bang for the buck. That's the desktop PC. I only use the phone to google something when I'm away from home, or to stream something on Spotify that I don't already have on my car's USB stick. (yay for unlimited data)
But here's the funny thing. What do you consider a "computer" these days? I have a desktop, work laptop, nexus 7 tablet, kindle fire tablet, kindle e-reader, work blackberry, iphone 5s, Roku and Tivo, all of which are really computers in the classic sense. That's like 9 computers (in various forms) for 2 people.