The raid is not loadout locked (confirmed by Luke Smith). Locked loadouts aren't meant to fix any problems imo. I think it's to increase the difficulty of certain activities like a Nightfall or Heroic Strike/Mission. Instead of switching weapons between every encounter to match shields or change from long range to short range, you need to choose a diverse loadout or communicate with teammates to form effective loadouts. It could turn out to be interesting, but I feel like it's stupid and unnecessary.