said by dnoyeB:because at some point you will want to use data and you won't be near Wi-Fi. And considering these companies are $50 per GB or more on people that don't have data plans I don't think you'll like you're bill when it comes in.
Whats wrong with having a smart phone that has wifi only? Ive done that for years. This is my first month having a data plan. And that plan is only $5 because I still mostly get my data over wifi.