...and with a 3G unit, you get that functionality at that time as well, only you can use 3G now when you need to.
The problem with the tethering debate is that there are two common use cases that aren't easy to differentiate in plans: Opportunistic Tethering, and Contingency Tethering. Opportunistic use is continuous, with no regard for the inherent limitations of a wireless utility. It becomes a home or office internet substitute, and leads to excessive use. Contingency use is for places between home and office without WiFi, or when traveling.
Contingency tethering should not be charged a $20 premium per month... $5 might be reasonable, but it really should be free.
Opportunistic tethering is quite damaging to the telco if the service is considered unlimited. It is unrealistic to think that this type of use should be free, even if you are within the 2GB limit, because you will saturate your connection when you do use it.