I've never used a screen protector on any device and haven't had a problem. Until a month or so ago. My HTC 8X suddenly has a 3/8" scratch down the middle. Don't know how/where/why/when it happened but I have it. Arrghhh!!
I figured if it could happen to my 8x then why not my SP3? So I popped for the Zagg Glass Screen protector. I've seen one before on a HTC M8 and I liked it. It cost $50 at Best Buy (love them by way, other threads be damned). It cost another $7 for them to put it on. I love it! You can't tell that it's on. It feels like the regular SP3 glass screen. Probably because it's glass. The pen feels normal. I have no issues writing with it.
Zagg marketing claims that if you shatter the screen you will only shatter the glass screen protector layer. Supposedly, the layered glass screen protector will shatter and prevent the actual SP3 from shattering also. I don't care to test this.
I would recommend a glass screen protector to anyone who wants to feel more safe and still have the same feel/functionality of the original SP3 screen.