I have never tethered anything. I think you need an additional plan anyway on AT&T to do so, not sure. I do have a question about tethering though. if your iPad is tethered to your iPhone it thinks it is on wifi, right? So is there any chance that it might do some syncing in the background like photo stream, for example, that it normally wouldn't try to do when not connected? Might run up your data charges unexpectedly. I'm not sure how it works exactly.
Most of the new Data Share plans (like on AT&T and Verizon) include talk, text, data AND tethering. But yes, it'll consume data from your pre-selected data package.
I believe that Verizon still offers free tethering from the iPad even without the Data Share plan...or not...the dang data plans keep changing.
The best way to tether is to jail break your iPhone (that is grandfathered with unlimited Data) and hope that the network doesn't threaten to throttle you...unless you have Sprint, which gives unlimited data.
Regardless, I would think that the vast majority would want the potential to use cellular coverage even if they don't use it very often. I can say that almost all of my friends (all 5 of them), who bought WiFi-only regret not getting the cellular model.