Then why would you pay for data in the first place?
As mentioned above, for when I cannot access wireless from home, such as in trains, outside or at my workplace. That's the price to pay for siri, the flexibility of being able to check emails, have the web at your fingertips and access location-based services, as little bandwidth as they consume. Heck, even my wireless at home can be cranky at times, my iphone can fail to detect it even though I am just 2 rooms away.
2nd, because I bought my iphone at a heavily subsidised price which comes bundled with a 3G data plan.