3G is a no-brainer if you have coverage, and also don't have a cellphone data plan. I was very happy with the web browser when I tried it (basic web, images turned off, mobile sites only).
Like if you're traveling and want to look up something. Or vacationing where there's no wi-fi. Or for a poor man's map, you can use
Kindlemap,net or
Google Maps for driving directions.
A couple of years ago, we'd have paid many times that extra $50 for lifetime (we hope) mobile 3G internet.
Oh yeah, there's also ordering books without a computer.