I bought the 64GB AT&T model for a few reasons:
1. I have a grandfathered unlimited data plan on my iPhone and don't want to lose it due to tethering. I use a TON of data on my iPhone and the thought of having to pay extra isn't pleasant to me. Also, as a previous poster said I'm not going to subject myself to a jailbroken device just so I can tether.
2. Overseas sim card usage - swap the AT&T card for another provider when traveling. Also helps with resale value when iPad 3 comes out.
3. GPS - yes, you have gps on the iPhone or via tethering but I like having dedicated gps in the device itself. I don't want to have to drain my iPhone battery in order to get gps data.
4. Battery life - tethering drains both devices very quickly.
5. Resale - also mentioned in point 2 - having sold my 64GB 3G iPad 1 for only $50 off purchase price I think I'll get a better sale price than a wifi only version when it comes to upgrading later.