The whole screen protector debate is a double edged sword. There are pros and cons to each, and neither is perfect.
1. While you might not scratch the screen, you will wear away the oleophobic coating on the glass. I've rocked a SP on my N4 since day 1, and since it leaves a sizable gap around the edge of the screen, I can see where the coating is now missing on that unprotected part. When I do remove the SP every now and then to replace it, it's amazing to see how easily finger oils stick to the outer edge of the screen and not under the protected section. I can almost feel a lip with my fingernail. You might not notice the difference over time, but I'd rather maintain that coating, and should I wish to go naked one day or one week (which I often do), it looks and feels like a brand new device. I also don't baby my phone, keeping it in my pocket all day at work in a manufacturing shop with a lot of dust and debris in the air, and where I'm constantly moving.
2. Sure, the SP wears out quicker than the coating does, but they're replaceable. I rocked my Gnex naked for a year and it has a myriad of small scratches and one or two deeper ones that are noticeable with the screen on. Wish I'd protected it, but the curved screen just made dry SPs look ridiculous, and I don't use wet ones.
3. Sapphire screens may not ever come to fruition, and if they do, there will be additional drawbacks. Sapphire is extremely hard and resistant to scratches, but it's also very brittle and susceptible to shattering. It works for watch crystals and iPhone home buttons, because they're relatively small and can be made extremely thick. But a phone display is much larger and thinner, and I don't know how manufacturers will overcome that.