I don't think this is a good idea.
For AI it would be just another variable in the overall formula of difficulty, which would make making balanced missions even more harder.

Also I think weapon dispersion is in the core of gameplay of ArmA 3, I don't want it to vary from mission to mission and from server to server. It's better if people know what to expect and that the dispersion is a constant set by BIS designers.

TL;DR: I don't think it's a good idea.

Edit: Improved wording

How would adding a new option make mission makers jobs harder?

And it's not like everyone would use this. It affects the unit and all of their weapons, so the intended use is more for AI.

Because it would be a redundant command. There already is the accuracy modifier for AI, why would you need a second one?

its not a redundant implementation if it also applies on the player!
currently, im in need of such a function
it is also possible to do a workaround but ... thats not the same as if i just need to call a single line of code

@Xendance: "I don't want it to vary from mission to mission and from server to server." My comment here is directed at you:
http://forums.bistudio.com/showthread.php?160330-Scripting-Discussion-%28dev-branch%29&p=2567592&viewfull=1#post2567592

That still doesn't explain why we should have a function that controls dispersion of weapons.
I think the consensus among the community is that dispersion should only be affected by the weapon in your hand. So I think that leaves affecting players out of the question, which leaves us with the matter of how this affects AI.
And as this only affects accuracy, it would basically do the same as decreasing accuracy and other aiming parameters in the AI skill array.

Wouldn't it be better if the dispersion was just a constant variable in the weapon class?