In part because an USB port is suppose to supply power. If trying to maximize battery life you don't want other devices bleeding power off your device. On the physical side, going to impact thin objectives if put a "normal" USB socket on the iPad.
If you have multiple devices want to hook to several devices then a MiFi solution would work better. The USB dongle is plugged into the MiFi device and multiple devices can sip the internet over that connection.
Bluetooth is capped at 3 Mbps. You ask same question about buying at 7 Mbps service and then funneling it through a 3 Mbps modem. You can but why.
Given ATT seems to have trouble with traffic they have ... tethering is only going to bring more bandwidth hogs to the table.
Don't have to. Could go MiFi and use the lower cost of the WiFi iPad to pay for it.
Uhm, no. Lack of a USB port on the iPad has nothing to do with power or the size. Its thickness and battery could easily accommodate the port, just as the MacBook Air does. The problem is more likely one of designer vision. They simply don't want to deal with people plugging in all kinds of hardware into the iPhone/iPad OS for many reasons... security, OS footprint, support, etc. And, of course, they're partnered with AT&T, who would much rather sell you yet another monthly fee to get online than use one you already have.
As for your MiFi suggestion, that would only be practical at home, which is exactly where everyone does NOT need it, since they'll be using WiFi at home. And no one is going to tug around a MiFi and plug it in so they can use a portable wireless device. That's insane.
As for tethering, there is no way the iPhone is getting faster bandwidth right now than 3Mbps over 3G. You would notice NO speed difference when tethering over Bluetooth. And since AT&T has already promised tethering will happen, it looks as though this may be the practical way to get an iPad online outside WiFi coverage without getting yet another AT&T bill.
So you're basically wrong on all your points, but I thank you for getting me thinking enough to provide my own answer.