Long story short? The glass in modern smartphones is remarkably strong, so unless your phone leads a very rugged life, you can probably get away without an annoying (perpetual bubbles and dust trapped underneath) screen protector.
Plus, if the phone does drop on the floor and the screen gets damaged, it could be as simple as taking the phone to a professional store, as they will be able to fix phones for cheap. But this then may make someone think twice about whether adding a screen protector would be beneficial.
On the other hand, a good screen protector is a remarkable shield against things like power tools and multi-story falls. Read via Yahoo Upgrade Your Life.