Horrendous cost aside, where I live (and I'm betting a lot of other places), you can't have multiple devices on the same data plan.
Why can't you have a 3GB plan shared by your 3G phone, 3G tablet and 3G enabled laptop? You're the same person, they're all your devices and your usage varies between the 3, but they want you to instead buy 3 separate smaller plans which total up to a lot more money for the same amount of data.
Sort that out and maybe I'd start considering 3G important in other devices. In the meantime I'll tether. Slightly less convenient, but a hell of a lot cheaper.