That's designed to make it look more like a movie since film has blur to that effect. Humans don't really have a measurable framerate so getting it look blurry with fast movement is more aimed at recreating movies than real life. I like motion blur in games (especially Source games) as long as it's not over done.
But when you turn quickly in game, you don't everything and because it is too fast too see everything it looks blurry anyway. You can catch a glimpse of something, as you would irl, unlike with post processing where you would see nothing.
I get what you are saying, but a movie it is necessary, you have to put the effect there for the viewer to appreciate it. In game, you control the movement and how you move will effect what you see. Apart from cutscenes, which arma does not have, there is no scripted scene like a whole movie, it is all dynamic, rendering the concept of including post processing useless, it just adds to load, makes controls feel less natural and makes the game look like crap
=EDIT=
Well, even my 7970 is brought to its knees by the game... I can play it on high-max settings @ 1920x1080, but I can't max it and have it with stable frames. But oh my days, even at high it is the most gorgeous game I have ever seen, it is mind blowingly gorgeous
=EDIT==EDIT=
Scratch that, I can play on full settings on single player, and it looks lush, but not multiplayer. Very strange...
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