You can't hold a Smartphone or netbook like a book. Screen is to tiny to share with an phone... and as it is, 4" screens are a BIT TOO big for a phone. trying sharing photos with family with a phone... like with 4-5 people. Netbooks don't rotate their screens. ie: flip a tablet to another person's angle and the interface flips right-side up.
I own a desktop (typing on now) a Thinkpad (for out in the field work as well as some home stuff) and an ipad(1) for work, presentations as well as around the house. Yeah, sometimes *I* don't want to sit at my desk with a 24" screen. I can lay on the couch or in bed and read, go to websites with an iPad / tablet.
Netbooks / notebooks have full blown OSed, you spend 1-2 minutes booting up. Watching TV, see something of interests and want to look it up? Okay, go to the room with the computer, boot it up, etc etc... a tablet powers up in a second (the way my Android phone DOES not) and it'll lock onto my network in 2-3 seconds and I can do searches.
Not everyone needs a tablet... heck, nobody really needs one. They are handy devices, like a cell phone, or any other device.