3G data comes in handy on my iPad. I have activated the monthly subscription twice in the past nine months.for those wanting built in 3g - really??? Kinda seems like putting a CD player in a car. Sure some people might use it, but 3G is so darn slow I hope I never have to use it. I'd rather lug around a 4g hotspot or pay for personal hot spot service on the iphone until 4g service becomes more prevalent.
Note that 3G isn't all the same. There's HSPA and the speedier HSPA+, as well as EV-DO (what Verizon and Sprint offer as 3G).
Some mobile operators run very speedy, reliable, and well deployed 3G networks, although admittedly, they are mostly in Europe and Southeast Asia.
So yes, there would be some benefit to having 3G on a notebook computer such as the MacBook Air.
Whether or not someone using it on a North American cellular network can exploit the full value of 3G data is a separate discussion.