I haven't used my data plan in awhile- but I'm pretty sure I only paid for the month if I used it. In other words if you can get through a month with WiFi- you don't pay. If you find yourself in a pinch and flip on data- you pay.
It's a matter of preference and you need to make the call. You can always try not using a data plan and seeing how it works for you if you're undecided. Polling others isn't the way to find your answer.
Personally, I find the data plan useful despite the ubiquity of WiFi. As with any subjective matter, YMMV.
That's why I buy my ipads wifi only...I have an iphone with a data plan, and wifi just about everywhere I go...On the rare occasions I don't, my phone is there..I'd say save some money and stick with wifi.