We tend to allow shields, simply because gameplay at Grim Up Nerf is so fast paced and our "dialect" of Nerf War is heavily biased towards overpowered auto and semi-auto flywheels.
Having a shield ends up as a distraction from things around you and as soon as you duck behind it, one or two players will be maneuvering to take you out.
Additionally, clusters of players behind a shield hearer become and easy target for anyone with a full mag.