No....
Besides, you can use your smartphone to tether.
You can go to library, schools, work place, restaurant and etc to use their Wi-fi.
Unless you are using your laptop at the parking a lot or the beach, 3G laptop is not 100% necessary.
It doesn't have enough markets for it, so that must be why apple isn't creating it.
Well, here in Sweden open wifi spots aren't that common but 3G works great and and is affordable. You come across students using a laptop with a dongle attached every day. The situation seems to be the same in the larger part of Europe, so I'd guess it's really the US markets that pose that kind of problem of several standards within one market (?).
I've been on 3G wireless internet for over 4 years and counting and unless prices go up drastically I don't plan on changing that. Wired home internet would cost me about the same or more (of course for more money I could get 200MBps wired, but I'm fine speed wise, anything from 5-20MBps, average between 8-10) but doesn't offer the same freedom.
An internal 3G modem would be great for me, though I don't know if carriers can/are allowed to lock it down here. That would be problematic if true. Also, LTE coverage is growing and several carriers are offering plans but it's not really there yet (also I hear the current chips like to drain batteries compared to 3G?), especially since the current speeds of 3G are quite nice here.
Then again I know that our 3G network is so heavily used that people get dropped calls due to the recent popularity of 3G 'home' internet (and chatty apps). So maybe 3G dongles are the way to go until LTE becomes mainstream (and less power hungry) after all.
Oh well, whenever the iPad (or its spiritual successor) gets a good latex-release that allows for on-device compilation and a capable text editor, I'll go for that as my one and only device/computer and stop dreaming of a MBA with internal wireless modem.