To expand on what @hug_of_war said, would you carry an iPad on your hip?
Smartphones, tablets, full-on laptops, and desktop computers all serve different purposes. The iPad cannot substitute for a phone (it’s too big) or a real computer (it’s too slow/weak), so it really is a niche item. Granted, that niche is a popular one, but try playing Crysis on it, or carrying it at-the-ready in a hip holster and you will see that it (and any other tablet yet to come) is not a panacea.
When I need new steel-toed boots for work, I do not by a toaster-oven, even though both have metal in them. When I need a car, I do not buy a sofa, even though both have places to sit.
Think about what you really need, and then make your buying decision accordingly. That is why I have both a cellphone and a laptop. There is no single device that can cover all of my needs. I do not want to carry a phone that can display an 8–½“x11” page at full width in a legible size, nor do I want a phone that I cannot fit into my pocket.