There's a couple of reasons, but damage is the main one, yes. The "which Soul should I use?" thread should list Herc at the moment, and once I get to update it then it'll also list Shingen's good sides. Well, Shingen's good side is basically her new EX skill which guarantees a PF Full Burst every 7 turns with zero help from other Hime. Realistically much faster than that, though.
But this is a pretty good way to put it though:In other words, running Herc PF or Shingen new EX might be the optimal way to play, but that assumes that you're running a team who are basically perfect by themselves. If you don't have that, chances are high that you need to run Herc with whatever EX completes your team, since Herc is just stupid good.