On Kindle Touch, 3G browser access is limited to amazon.com and wikipedia.org. That said, I think it is totally worth paying for the 3G model (though I got mine for only $110 at Walmart).
It is not just for Kindle Store/library access: all of the internet-enabled reading features are available whenever 3G is (sync, Translate, wikipedia lookup, rate this book, report content error, Share). I use wikipedia lookup a lot (the article summary pops up just like dictionary definitions, no need to launch browser unless you want more than the summary). It's a pain to connect to wifi sometimes also, and KT will not connect at all to one wifi network that I spend a lot of time in.