I think The carackter of an army depends most on the skills and the charisma of its leader...If there is an ambitioned warchief, who tries to enfirm his power, the "quote of organisation" might be higher than in an warband following an rouge, just to make some cases and plunder some villagers...
I will use my Roman Legion for historical wargaming, but for fantasy too, the "Legio-Style" is it, what makes the army cool...so why don' try some skirmish with some Chaos-Barbarians or a bunch of Orks...
In Warhammer Fantasy I'll use 'em as an early Imperial Army...so what, who tries to stopp me.