From where I'm sitting, computing works best with a 3 tiered approach.
1. Primary computing from a desktop computer for all sorts of base station tasks, such as file archiving and downloading or gaming.
2. Secondary computing from a netbook for tasks that require the flexibility of windows or linux on x86, but with the portability and safety of a 250 dollar netbook instead.
3. Tertiary computing from a smart phone for tasks that don't require great flexibility or controls.
Even though I travel a lot now, I still find this paradigm applies. My base station is still great for having physically secure, reliable, safe storage and high powered processing power; my secondary computing comes from one of the new breed of laptops with netbook level processing power (full size laptop with an AMD c50 apu), but only because I spend so much time away from my base station. Despite having great uses for both of my computers, my mobile phone is perfect for a lot of times I won't have a real computer in front of me.
Netbooks and tablets both attempt to fill the niche of secondary computing. Most people can't afford a desktop replacement for secondary computing, or they don't want to travel with an expensive desktop replacement, or they can't handle the size or power or heat of a desktop replacement.