If you really want to hide behind something, stick out your wrist and spray your Uzi at the enemies general direction, it's -6 because you don't see a thing and -2 for shooting from cover.
If you're hiding behind something, pop out for a second, take a shot or two and then duck back behind cover, you take the -2 for the quick movements and somewhat unsteady aim, but gain the benefits of good cover. (+4 Defense)
Problem with the Attack penalty is that there isn't a distinction between Good or Partial cover, imo. It would be better if the -2 applied only to Good Cover. You'd have the option of trading 2 attack dice for 4 defense dice, or keeping your dicepool, but leaving a larger part of your body exposed and making you easier to hit (only 2 extra dice for cover).
Let's say you're standing behind a waist-high brick wall and your enemy is 20 meters away shooting at you.
You'd have 3 options:
A. Lie flat behind the wall, stick your gun out and pray you hit something.
+Good: Enemies will have a really hard time hitting you. (They get at least -6 for not seeing you and will have to bring heavy enough firepower to shoot through the wall.)
-Bad: -6 attack for Blind Fire, another -2 for firing from cover.
B. Crouch completely behind the wall, pop up to take a shot or two, duck down again, relocate, pop up to shoot then dive down again etc.
+Good: Enemies will have a harder time hitting you, as they have to be quick enough to aim and fire when you pop up. (+4 defense for Good Cover)
-Bad: -2 Attack because you never really get time to aim = attacking from cover penalty.
C: Crouch behind the wall, elbows leaning on it for balance while you shoot your enemies.
+Good: Since at least part of your body (everything down from your chest) is behind the wall, enemies have a tougher time hitting you. (+2 defense, Partial Cover)
-Bad: No penalties for you other than the fact that you're easier to hit.
From your question, I think your problem is that you're getting lost in the whole time-paradox issue of turn based combat:
While it's true that at the table actions happen in turns, to the characters everything happens simultaneously.
So in above example, let's use option B.
At the table, GM says "Enemy shoots twice and misses both, Zilfer's turn. Your turn comes up and you say, "I stick out my head, fire twice and duck down again". Enemies turn comes up again and... You're out of sight behind a well! So he takes -6 for blind fire and has to try to shoot through the wall? NO! The time in which he preforms his actions is the same time in which you preform yours.
What happens in the game world is this:
"Zilfer pops up from behind the wall. Enemy swings his weapon in your direction before you get off a shot but was too quick to fire and misses. Zilfer returns fire and hits Enemy in the shoulder at the same time as enemy gets off his second shot. The impact screws up Enemies aim, his shot chips off some brick 20cm to your left. Zilfer quickly get off a second shot and dives back behind cover, preventing Enemy from getting a better aim at him."
My examples however aren't RAW because the Attacking From Cover thing doesn't differentiate between Good or Partial Cover.
By RAW, both B and C would cost you 2 dice on attack, but B'd still give you better cover than C, so always do B.
