This is the way I look at it....
I was recently in San Francisco for several weeks, staying at one of those hotels that charges an absolutely ludicrous amount for internet access. I am not a big mobile browser...I like it for quick searches and checking my facebook or whatever, but I can't surf for a long period of time on an iPhone.
Since this was before easy tethering without jailbreaking (and I didn't feel like jailbreaking), I went ahead and got myself an aircard and signed up for the $60/month service. I figure with the amount of traveling I do, and working in areas without wifi access, it would almost pay for itself.
Let me assure you that the service on the AT&T aircard in the Union Square area of San Fran, even sitting by a window facing pure openness, was TERRIBLE. Unless I held my laptop at a certain angle in the window, I could barely get enough service to even read email. That is hardly worth $60/month. Luckily, they have a 30-day return policy, so I used it for my few weeks in San Fran, will pay my data charges, and have already returned it.
Now, every now and then, I need to download something to my laptop while out at work. I have no interest in torrenting, streaming video, watching my sling, or whatever over a cell connection, so I won't be abusing the network at all. Since there are only three options (no access, pay $60/month for bad access, or be sneaky and tether a little bit), I chose to tether for the few times I need it. I would GLADLY pay and extra $10/month for it...but not $60/month when I will barely ever use it, and only for light use. Offer some tiered plans here.
So yeah, I don't agree with the "I have unlimited data and I will use it up however I feel", but I also don't agree with "the only way you are allowed to do this is by doubling your bill".