Yes, you would have to pay the $30 per month data fee on that line also. When you accept the terms, you are agreeing to the data plan for the life of the contract, even if that phone is switched to another number and replaced with one not needing data.
I have three lines on my account:
Mine, a BB Tour, ineligble;
Wife, a BB Curve, eligible;
Mom, Dumphone, eligible.
At one point VZW had a BOGO on the Moto Droid, and I considered upgrading at the time, using my wife's and mother's accounts to do the BOGO. I could have done it, but then my mom's line would have been committing to the data plan even though she would be switched right back to her flip phone. Needless to say, I did not want to pay $30 a month for data on a line that could not even use it.
I ended up upgrading to the Incredible on my wife's account. Her line was eligible, and already had data on it, so there was not any increase in cost.
Hope this helps...