Why do you think it's useless?

I was focusing on the fact that I believe that it is not necessary to have 3G built into the devices themselves for ubiquity. But having the ability to add a dongle is certainly useful (and perhaps required). In fact, it is likely now much more useful then built in 3G since it is easier to transfer a dongle from one device to another than it is to transfer a SIM card.

No doubt that tethering is not perfect, but for many uses it beats and is more flexible than a second data plan. It inherently serializes all of your devices through one point (which you seem to agree is important). Don't forget that tethering can also be done through USB, saving the energy of wifi usage all while charging the phone. In this way, it would turn your phone into the USB dongle. :) Not to mention that any current inconvenience of tethering (phone calls) is something that could be easily overcome if tethering were seen as a "good thing" by all parties.

Another one of your points is good: carriers in the US do have oversell issues, and they seem to currently hate tethering for this reason. However, once they realize that people will need access with more than one device and that most will be unwilling to pay for a plan per device, tethering will become the simplest solution due to its built in serialization. Without tethering, in order to offer a multi device per person plan (which I am claiming is paramount in the very near future due to multi tablet per user ubiquity), the carriers will need to implement a physical or network level serialization solution across devices. This is not likely easy and I suspect would take at least a decade to implement. Instead, I predict that the carriers will wake up to the realities (and simple benefits to them) of tethering (although, perhaps a bit late in the game). They will have to charge a bit more per plan and increase their network capacities and take advantage of the builtin serialization in tethering.