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.

I have T-mobile. I have unlimited tethering but it’s only 512Kbps for tethering, which is surprisingly enough for light dev work in a pinch.

The $130 extra for an iPad with cellular and $20 extra per month for unlimited data on the iPad is well worth the convenience of having two cellular devices.

I pay roughly $210 a month for five phones and two iPads all with unlimited data.

T-mobile also has free data roaming in most other countries where we go on vacation.

Tethering incurs extra charges too.