Grabbing a shield shouldn't have "no effect," it should definitely protect them. I just don't think it needs to be +1armor automatically. Ask how they're using it, what it's doing. Make them act under fire and if it works, take less or no harm, situationally. If it's a huge riot shield and they're moving in, maybe they're totally protected until a certain point.
Basically, make it count within the fiction but avoid a static +1armor, that's what I'd do.