Someone like Susanoo still fulfills the role of DPS. That's still a use for the team.
Baldr doesn't do anything for the team. She doesn't output damage like an attacker would in her spot, or someone placing DoT effects would. She isn't raising the team's DPS through attack buffs or defense debuffs. She isn't disabling the enemy. She doesn't reduce incoming damage to the team via defense buff/attack debuff/team-wide damage cuts/covering. She doesn't heal the party. She only does self survival, and that is practically useless without the ability to force the enemy to attack her.