We haven't seen much of Shouheikun's actual experience in the battlefield other than that one appearance of him near the end of the coalition war.
But from his experiences in developing defensive strategies and scenarios in the King's courtroom suggests that he's pretty much a pure strategical general, not unlike Riboku...just with less actual battlefield experience.
Too bad we haven't seen much of SBS either other than that one battle where he managed to get close enough to Ousen's location and it caused Ousen's army to get pushed back, making him lose that battle.
The remaining Kan Ki army might be able to formulate strategies that work more effectively in the dead of night (such as tampering with food supplies, arson, etc) but with Kan Ki's death along with the rest of their bandit higher ups, this isn't even a guarantee at all.
Shouheiken may want to have Shin and his army in this fight because he might be the only one (Ou Hon is debatable, but I doubt he's on the level of someone like Ou Ki yet) that can hold back Shibashou 1-on-1 due to his experience going up against fighters that are supposed to be above his own weight class.