I really don't see a reason "not" do to bluetooth if you have it other than the fact the the battery drains when not hooked to the usb cable.
But you can run bluetooth and hook it to the cable too to charge the battery!
You may say, "then why not just use usb tether?".. because its not really any faster.
At least with bluetooth, I can break away and take my phone somewhere else in the room "and use it" without killing my internet connection.
As stated before, the only "main" reason to use usb tether is if your accessing wifi, then you get the wifi speeds and will be better...
but if I have wifi.. why do I need to tether?.... help me out here?
I live in an outlying city on the fringe of good stable 3g connection and here are my results through
Speakeasy - Speed Test
Bluetooth with 3g only:
1st test = .88mbs download, .42mbs upload
2nd test = 1.23mbs download, .63mbs upload
3rd test = 1.56mbs download, .53mbs upload
USB tether with 3g only:
1st test = 1.74mbs download, .71mbs upload
2nd test = 1.41mbs download, .63mbs upload
3rd test = .99mbs download, . .57mbs upload