"If a company has a device that doesn't support tethering, why would you buy their products if you want to tether it?"
Because almost everything else about the iPhone outweighs this one issue.
"If a network does not support tethering for your particular device, why would you join them if you one day hoped to tether?"
Not that I have a choice in the US with the iPhone, but if I did I would still have AT&T because:
1) Roll over minutes and in-network calling have resulted in a cheaper plan (than what I had with Verizon).
2) Customer service is much better (IMHO).
3) You can do voice and data at the same time.
4) GSM makes it easy to use my phone internationally (and cheaply with rented GSM SIMS)
5) Faster data speeds (in my geographic areas).
6) Dropped calls etc... hasn't been an issue for me (again in my geographic areas).
7) Free connectivity at AT&T hotspots, like Starbucks.
"Stop hoping for half-arsed solutions, trying to "jailbreak" your phone to do that, etc. "
There is no "try" there is only "do", and it's not half-assed. I have no problems with jailbreaking iPhones. It's always resulted in an iPhone that just worked better and enabled more features and functionality, including tethering and tether-sharing. I see no reason to ever stop, unless Apple eventually enables all the reasons why I jailbreak.
"And, besides, phone tethering is old-hat anyway. It costs literally a few pounds / dollars to connect a PC to a 3G always-on connection on a decent tariff in the country of your choice."
Ya...I'm not likely to change countries based solely on the price of 3G cards/dongles. Here in the US, with any carrier, 3G laptop service sucks on a price:utility ratio.