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You only want to use them for the fun factor or as morale boosters ( but if you need more morale you are doing something wrong ) as they are very few in numbers and their upkeep is extremely high which doesn't make them be worth the cost and the hassle. Yari ashigaru will always be the main component of your army cause of their low cost, high numbers and cause of the possibility to put them in wall formation when needed, if you want you can mix in some samurai units but not too many unless you want to see your finances sink. Yari wall makes yari ashigaru absolutely op and that is a fact, yari sam can not compete cause they have no such formations or buffs of other kinds to make up for it and naginata sam more or less cover the same role as yari ashigaru when in wall formation while at the same time being stronger at the cost of a much higher upkeep.
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Historically speaking spears dominated the battlefield, the katana or any kind of sword was more of a side-weapon you'd use in close quarters.
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