Because you are changing the center of gravity. If you have your weapon close to your body. You are able to turn more efficiently. Spend less energy on counter balancing.

Holding it infront of you while running makes you spend more energy counterbalancing and you are not able to stop/accelerate/turn faster.

@ProGamer,
It is sort off in the game, But its unbalanced.
I.e if you run with combat pace+sprint disabled and weapons lowered.
You will slowly regain stamina, But the animation is off. Its more of a fast walk then a slow run.

it also doesn't add up. why run with your gun in one hand diagonally across the body instead of either besides you or holding it with two hands.

But the moment you enable combat pace it drains stamina.

Their is no balance. Its either move incredibly slow and awkward or sprint and drain stam.

Made a addon that sharpens up the Deadzone.
Def try it out: http://forums.bistudio.com/showthread.php?163285-A3-FreeAim-Fix

I think it was fixed maybe 2 or 3 dev builds back. But last Spot rep made it official. http://beta.arma3.com/spotrep-00008

"Removed sensitivity smoothing for aiming deadzone"

Aiming Deadzone has been fixed as of latest patch.

@Concurssi,

*wakes up*
Look at https://www.youtube.com/watch?v=dGc09mCmUVY, It "fixes" the exact issue "dslyecxi" writes about. Everybody knows aiming deadzone in its current state is not 100% realistic. Thats why we have this ticket.
His opinion doesn't matter on this issue for that he tries to disregard a whole game mechanic due to it not working correctly besides trying to fix it.

With the amount of viewers he generates - he could have resolved this issue along time ago.

After thinkering with the controls i found a solution to make arma behave exactly as explained above only with minor bugs seeing i dont think its meant to be used like that.

My video reviewing it.
https://www.youtube.com/watch?v=dGc09mCmUVY

Found a old thread that maybe explains it a little bit better.
forums.bistudio.com/showthread.php?72080-Next-generation-freeaim-or-floating-zone/

Temporary FIX regarding aiming deadzone for Arma 3
https://www.youtube.com/watch?v=dGc09mCmUVY

Temporary FIX regarding aiming deadzone for Arma 3
https://www.youtube.com/watch?v=dGc09mCmUVY

Temporary FIX regarding aiming deadzone for Arma 3
https://www.youtube.com/watch?v=dGc09mCmUVY

Temporary FIX regarding aiming deadzone for Arma 3
https://www.youtube.com/watch?v=dGc09mCmUVY

Temporary FIX regarding aiming deadzone for Arma 3
https://www.youtube.com/watch?v=dGc09mCmUVY