They are the ones most likely to spot signs of an IED or possible ambush.
I'd have to disagree with this. Generally, the gunner should be down behind the turret armor to avoid being the victim of an IED. The driver and TC are responsible for spotting IEDs, the driver scanning the TC side, the TC scanning the driver side. The gunner only pops up as you go through major intersections to stop traffic. His primary duties are providing security while stationary, and engageing VBIEDs as necessary.

That "pope glass" is going to get scratched up, reducing the gunners ability to see detail.
We had some HMMWVs use that glass on their turrets, usually layered on top of the metal to provide double protection. It works well. I also had it around the BC's position on my two Bradleys. It does get a bit scratched and dusty, but the loss of visibility wasnt too bad. And it did allow you to expose yourself and see more without having to worry as much about having your head taken off by an IED, or sniper.