The size advantage of the Nexus 7 is great for mobility (honestly dislike carrying an iPad in my bag) but the majority of areas are not going to have wifi, hampering a bit of it's advantage.
Most people who buy tablets are likely to have a smartphone so it would seem more appropriate to have tethering and use that one connection for multiple devices.
I realize that's a personal choice, but I currently own an iPad 2, Kindle Fire, 2 Kindle Touches, and I just bought the Nexus 7. I'd rather pay for one connection and tether all those devices to that connection than each one have their own.
For you.
For people like me, it IS a plus. That's why Apple makes two versions of the iPad. Not to mention tethering drains the battery.
Also, there are more popular 4G data plans now, such as T-Mobile's "No Annual Contract Mobile Broadband Passes" that you can enable on-demand, only requiring once-year usage to keep the account active. I'm not sure if other carriers offer similar plans.
I used to think that has something to do with stupidly expensive and restrictive US data plans, but you have some decent ones. The situation in Europe is still better, though.