not everyone needs to contribute to the team: Susanoo for example does nothing for the team, she just kicks ass and chews bubblegum. Glass cannon, that murderises everyone but does nothing for the team. Baldr doesnt do that much damage, but she has 2 different ways to negate damage, which, combined with some buffs/hero skills can make her practically unkillable. Yes, she doesnt do much for the team, but since Baldr alone is pretty much immortal, that means that the rest of the team can ignore her and focus on keeping the rest of the team alive and if enemy hits Baldr, chances are it will be reflected/negated or just brushed off, considering her huge hp pool (for SR tier). Effectively, if enemy hits all members of the party equally, having Baldr is like taking 20% less damage overall. Amon, on the other hand has VERY niche uses... So niche, that you might as well call it "useless"...
and another argument in favor of Baldr: she is cute XD