Binoculars would obviously let you see farther, but they would also let tag blocks with a color. This color only shows to teammates. Toggle binoculars=B, zoom=scroll wheel, tag=left click, clear tag=right click, change tag color=arrow keys, toggle tags=N. There should be an offset between gun crosshair and binocular crosshair when switching between them to avoid sniping with them.

Good for marking tunnels and other hidden structures. Maybe they could reveal enemies they're aimed at too.