I don't know about "most". But the tempered glass screen protector is worth the $6-8 to me because it is simple to install. Wipe your screen, line it up, drop it on. Since it's not flexible (but the thick adhesive is), any dust just gets absorbed by the adhesive and there are no bumps or air bubbles. Didn't come out right or a big piece of dust? Just lift it up and redo it (if there's just take it off with tape). SO MUCH EASIER than the plastic ones.
The bad: my GF cracked hers within a few days. It was a Zagg for which I paid $17, and after looking in to their "lifetime warranty" and having no luck getting my password set on their site (I checked out without making an account), I decided just to go with cheap ones from Amazon. They work great.
But it's not going to protect your screen from more than scratches.
I disagree - I find installing the plastic ones much easier! I put a TechArmour glass one on my phone last week, and although I wiped it until it was spotless and even did it in the bathroom after the shower had been running, one piece of dust managed to get on the screen right in the middle. It's not very noticeable, but with a plastic one I could always put it on perfectly as any dust could be removed with a sticker by only peeling a little bit of the protector up.
I do like the scratch-resistance of the glass one compared to plastic ones though. I wouldn't get it if it was much more, but for the sake of a couple of pounds it probably provides a better experience.