Foot Knight is a tanky support who buffs the team with his auras and, as such, needs to stay close to them for full effect. Mercenary is a melee DPS whose kit demands he always be engaging as many enemies as possible. As such, Foot Knight’s Valiant Charge career skill is better suited for Mercenary and Mercenary’s Morale Boost career skill is better suited for Foot Knight.
Right now, we basically have the pro-DPS ult on the tank and the tank ult on the melee DPS. Giving Morale Boost to Foot Knight not only reinforces his role on the battlefield, it better fits the lore in that it demonstrates the bolstering leadership presence we’d expect to see from the career that most closely matches Kruber’s past as an infantry captain. Giving Mercenary Valiant Charge strongly reinforces his role as a frontline melee DPS and better suits his lore as a no-bs, aggressive merc.
At present, FK/Merc are both a bit eclectic and very clunky in how they handle and this change would really polish the character in general. It would help FK be a better tank and it would help Merc be a better DPS.