I have tethered to my cellphones for years. Starting with my sprint blackberry and then my Verizon storm2. The iPhone does it wirelessly with up to 5 devices for $20 more a month for the service. I started with 2 gigs a month and now I'm up to 3 gigs. I use it as a back door access to my terminal server if I have a system wide failure. A 3G service on your iPad or mini computer will cost you $50 a month while tethering it $20 a month for the same service. It is easy math
While I agree with your major point, the math is not THAT easy. For example, the 3G service for the iPad (on Verizon) is a 2Gig ceiling for $30 (not $50) and can be added/deleted on a monthly basis. Tethering is, as you note, $20 per month. However, if you have a grandfathered unlimited data plan, the carrier may not allow you to add tethering to that plan and may instead force you into a more limited monthly plan. It's not quite clear to me whether the tethering charge can be added/deleted on a monthly basis. (It may depend on the carrier.) If it cannot be turned off, of course, the $10 per month savings versus a 3G enabled iPad may be illusory.
As noted, I'm not disagreeing with your primary point. If you have a cell phone plan that supports tethering and you don't already come close to the ceiling for that data plan, you may well find it advantageous to share the data plan between your phone and your iPad. (In fact, Verizon has recently included a 2 Gig upgrade in the monthly ceiling if you tether a cell phone. This was the result, I think, of complaints to the FCC that Verizon was charging $20 per month simply to use data the consumer had already paid for.)
I've been wandering around in this jungle of charges, services, data plans, etc. for awhile now. The bottom line is that the most advantageous plans in terms of costs and benefits depends on the device (phone tethering versus mobile hotspot); data use (minimal to 10Gig per month); carrier (they're not that different but it's very confusing to sort out the differences); contract versus non-contract (i.e. add/delete service for a month); and the value one places on convenience. There isn't a simple single answer...sorry to say.