Lots of posts on whether or not to use a data plan. I've analyzed billing and discussed in detail with a few reps. Reason was they didn't add my data plan package, so I had to have bill adjusted.

Note, though, that data, when you don't have a data plan, is billed actually two ways it seems. You are billed by the minute AND in kb.

For example, I had a 16 minute data session, which went against my minutes. Though the 127 minute session was off peak so it did NOT go against my minutes (was included in the unlimited nights and weekends).

KB was also billed separately.

Another reason a data plan itself makes sense. With a data plan using data during the daytime does not go against your minutes.