People use Shingen and Herc as end game options, people who aren't strong enough have to opt in for other things. You use Joan or Mordred as a newbie, you use D'art on maybe some other situations that calls for it, Solomon with her MEX can sweep towers, gem time quests, dungeon crawling and Andromeda is also a core part of being raid support not just Joan. The only 2 i cant find a reason for most people to use are Sieg and Arthur. Why bother with Shingen or Herc if you are not a damage carry on high rags? You are not doing a huge part of the work so why not opt in for other shit that could help like Andromeda or Joan as you said.