Haha that's interesting!
But then, wouldn't the same logic apply to enemy morale?
"If the roll is higher than 10+HD, they flee or surrender at the first opportunity."
For me I would house rule hireling morale, because I feel that the whole thing about morale is having the grit to stand and fight when every instinct is to flee.
Instead I would roleplay those kind of "survivability smarts" externally to the rules, where a veteran hireling passes the morale check but elects to flee based on rational mind.
They'd do some fighting withdrawal and regroup, rather than panic fleeing