@DX- I thought the opposite- shields would be best on open ground where there is no cover. You carry the cover with you. Plus, if both teams have personal shields that don't completely cover you, then that seems fine, as kills can still be made, especially with flanking maneuvers.
Except that cover isn't for protection. Cover is used to either
A: Slow down the tempo - if the game is flowing too fast, it can be ground to a halt using chokepoints, trees, walls, and whatever to force more stationary, position-to-position fighting. It is usually easier to score kills when the enemy doesn't have full range of motion.
B: To conceal yourself while trying to flank, ambush, return a flag, etc.
I don't ever assume that it will protect me from being hit. Unless it's a wall or tree or other large, solid object, shots can be powered through. Cover is an impediment to agility, which is the best defense. A player who can dance their way to 0 deaths, war after war, has an asset more valuable than any shield or tree. In the open, I'd rather have the full range of motion and sight to dodge. A shield would just slow me down. It's already difficult enough to achieve full potential reaction time when carrying a big CPS gun full of water. Most streams could be blocked with a gun anyway - only the ranged riot blasts and other spreading shots are dangerous in that sense.