Nah. I have a 3G Kindle right now (2nd generation), I rarely use the 3G — just to download new books and sync occasionally. And I only ever use it because that device didn’t have an option for WiFi at the time.
Paying an extra $50 for 3G seems silly (though it is a good deal if you’re going to use it a ton since there is no monthly fee, obviously). The bigger issue though is that 3G drains battery life much quicker. That’s the big reason I turn it off by default on my current model.