There are plenty of reason to use other Souls then just Shingen or Herc. First of, if you dont have a team that has some decent speed, they are actually more of a burden or solo-actor when it comes to pacing.

Andromeda is even more important in True Ragnarok then Joan is (what use do you have for a random appearing dmg cut when there is no danger incoming?). Both together make the Raid inredible safe for everyone though.

Solomon becomes an incredable nuker with her reset skill, even outside of Tower. Dropping 2m+ damage in one turn can help out a lot on controlling bosses.

Hercs Overlimit makes her an incredible nuker and overall attacker in a couple turns already. With a mediocre setup I casually threw 1.8m Full Breaks around, not even mentioning her Burst damage that went to cap just like that.

Just some examples. Sure, if you have the perfect setup for a consistent 2/3 turn FB pacing, it makes sense to go for Shingen only.