Recently I've put to rest the G3 screen protector on the Treo phone.
It has served admirably during the last 2 years, and I think it can hold up for years to come based on my experience. I've been using the G2 since 2002.
I was interested in using the phone without a screen protector - The digitizer had to be re-calibrated and there's less parallax. But that's about it.
There were two issues with the G3 on the Treo phone. The first issue was that, the G3 slipped underneath the frame from time to time, enough to expose the edges ever so slightly and introduced dust and other small particles between itself and the screen.
The second issue was that, when I sweated a lot, or when my pants got wet, water accumulated between the protector and the screen. I carry my phone in my pant pocket.