Screen protectors definitely aren't necessary, but neither is health insurance, until something happens where you need it. I've had dozens of devices that I ran naked, no cases, no screen protectors, and nothing ever happened. Then one day, the tiniest bit of sand was in my pocket from a trip to the beach, and the things it did to my Galaxy Nexus screen were horrible. This was a phone that I dropped nearly $700 dollars on, and every single time I looked at the screen, I saw these unsightly gashes.
My only complaint with screen protectors is how awful they look and feel. The only options were pretty much cheap slick plastic that feels ok, but scratches fast and looks like garbage, or one of the gummy "apply wet" type protectors like a Zagg or Skinomi TechSkin, which do a better job of protecting the screen and holding up to damage, but dont hold a candle to the smoothness of glass, and often give an orange peel type affect. However, there is now the option of tempered glass protectors, which are nearly unnoticeable, feel just like a naked screen, and protect amazingly well. The price is quite a bit higher than the plastic protectors, but I think they're worth every penny. Xgear has one for the N7 called the Phantasm. I would recommend the investment to just about anybody who has concerns for their screen.