I have a few cases and wouldn't think of using it without one.
I love it when people say they don't want to cover up such a beautiful phone. Fine, but I didn't realize people sat around all day staring at their phone like a sensual object. Besides that, no one gives a rats behind what phone you have and how "beautiful" it is. People are getting carried away with this gadget thing. Pretty soon there will be cell phone beauty contests, right up there in importance with 4yr old girl beauty pageants.
Besides protection, I've seen some cases that actually make the phone look better in my opinion. And if you're that much into showing off your phone like a trophy wife, you could always get a clear case so the "beauty" can shine through and the world will fall at your feet for owning such a marvelous and breathtaking device. Make sure to have a towel handy so you can clean up the drool from everyone lucky enough to gaze upon the wonder known as NEXUS 6!
Carry on.