Always have a screen protector... I've used tempered glass and plastic (plastic ones have come a LONG way in the past couple of years btw).
The way I figure it... Yes, the glass screens are extremely durable types of glass that are resistant to a lot of the wear and tear of daily use. Still, one of the most common bits of material out there, sand, will do a number on your screen. Screen protectors are cheap, easy to install and even easier to remove.... and the plastic ones no longer have that plastic wrap like feel they had not too long ago (though glass is still better).
Last year, when I got back from a vacation, I found a wee piece o' sand embedded in the screen protector. It punched a hole in it that didn't mark up the actual screen, but had I not had a protector on the thing, it would have most likely done a number on the thing.