Until I got the smart phone, I didn't use the internet all that much. But I still had a 1mb plan for a couple dollars on my line (back when at&t bundled texts with internet), I never removed the data when I upped the texting to unlimited family. It came in handy like checking my email, and it's probably ok for facebook. But not too many sites have a mobile browser, and even then it doesn't show up well on a regular phone.
For the most part, you're not locked into added features like that. So you can add the unlimited data for 10$ (which is the price AT&T charges for non smart phone users), and see how you like it. If you don't think you need it, call and change it to a lower plan or cancel.
Anytime I have something on my line that I need to upgrade or downgrade, I just give them a call and they change it for me. Need international calling for a few weeks?? call and activate that plan and pay for only the amount of time you need it. Need unlimited texting one month but not all the time, just call and change your plan. They're even good about backdating it to the previous bill if you wind up with a larger than expected bill...