Simple: other devices. Not everything syncs via Bluetooth, WIFI or ANT+, etc.
I have devices I need to connect to my computer to sync data from. My Garmin Edge, my Nike+ Watch.
Luckily my Nike Fuelband and Fitbit sync via Bluetooth now. But until I can get away with running an iPad, Chromebook or Surface RT .. I will need a traditional laptop or desktop. Macbook Pro or Air, iMac, Windows Desktop or Laptop, or Surface Pro…
Another reason, but not a complete dealbreaker, is software and services that I can’t use on non-traditional devices.
Excel 2013 with Power BI? Can’t really run that anywhere but on a traditional laptop or desktop. Yea, I know I can remote into a machine or server, but that is cheating. What else? Visual Studio, Management Studio, etc. Same thing, I could set up some IaaS VM and do things, but I would be taking way longer trying to create from an iPad or Surface RT, etc. The Surface at least as a sanctioned keyboard. iPad with a bluetooth keyboard works well. Chromebook works, but when it comes down to hard core work, these devices fall down.
I don’t even feel like typing up a blog post on these devices. I can pound out tons of content on a laptop or desktop. On a tablet/limited device, well, you are limited. Limited by entry speed, limited by having to be always connected, etc.
I am looking forward to a convergence of laptop/tablet along the lines of Surface. Surface Pro 2 is close. Probably the closest device. Unless you want to develop iPhone apps, then you need a Mac too :)
Hoping in the not to near future I can limit the number of devices I need to create or do the things I need to. Content creation, app development, syncing external devices, etc. If 2014 isn’t the year, guessing 2015 will be.