Don't forget the other truism:
All truisms are false.
I think it's more important to realise that both gameplay and sensibility are important in any game that represents a situation, at least to many players.
A realistic mounted cavalry game could be fun if done well enough that there were ways to effectively play any force type offered to the players. Game designers who sacrifice realism as soon as they see any kind of difficulty seem pretty weak to me. After all, medieval knights were a very effective unit type for a very long time, for reasons which were very real. One doesn't have to nerf crossbows to make a game where the same would be true - it can also be done by correcting whatever was ahistorical about the game situation.
If one really doesn't care about realism, why even play representational games at all?
There are too many games out there where the design apparently starts out as "let's do a game about an interesting setting like X" and then as soon as anything gets in the way of fun, an arbitrary and illogical change is made, without even trying to find a logical change, even if all they'd have to do is ask someone who knows what the actual historical reason for something was.
In this case, there is no need to force missile units to have such a large chance of killing their own men. Even from a pure "fun" perspective, it's not fun to excessively kill your own men in silly circumstance, like when there is one limping enemy spearman running away and your archers kill a dozen of your own elite pursuers by taking pot shots at long range. That problem isn't helping anything.
The solution which has been suggested many times increases fun, reduces micromanagement, makes sense, is realistic and even historically accurate: Have archers only shoot when they will have near-zero chance of killing their own men!
It would also be possible to reduce the entire-army-missing spread of missile attacks, and have the chance of affecting a target in the hit square reduced - it would look a lot less silly, reduce unintended casualties, without making the weapons more effective than intended.
PvK