It drains the battery, for one thing. For another, it's one extra additional step that I have to go through to get online on the iPad, and the lack of friction

It drains the battery, for one thing. For another, it's one extra additional step that I have to go through to get online on the iPad, and the lack of friction to doing things online that are more substantial than is comfortable on a phone is the primary purpose of the device. Finally, I very often tether my MBP to my iPad, because the iPad's battery lasts much longer. If my phone battery is low I pretty much need to charge it so I can get calls from family. If my iPad's battery dies I'm merely inconvenienced for a while.