I just got a nexus s after a long while of wanting to jump on the smartphone bandwagon...I had an old flip phone before.
Given that I can't really afford to replace this one if it breaks and that I'm not someone who upgrades often (I never care about having the most up to date gadgets...as long as it does what I need), I didn't even hesitate in buying a case.
In my opinion, thin cases defeat the purpose. People either choose a case for protection, or keep it bare for the sleek look. A thin case gives you neither the full damage protection (maybe scratch protection for the back though), and it's still not as sexy as the phone bare.
I myself got a Seidio active case. Not the thinnest, but it does look good, good protection, and has good grip too. Just my 2 cents.