Imagine that your squad is hopelessly outnumbered and pinned down. Your only hope is to hold out until more reinforcements arrive. Fortunately, the enemy doesn't want to take any more losses so they stay back and you are in no immediate danger. Suddenly, one of those crappy Kajman gunships shows up to finish you off. Your only hope of survival is to shoot it with your anti air missile. You quickly pull out your launcher and start to get a lock on it. Right before you can get a lock, the enemy helicopter flies straight over your head and some weird force prevents you from looking straight up and shooting it down.
The current maximum angle for looking up or down is like 75 degrees, it should be 90 degrees so you can look straight up or down. There are many situations, like the one described, in which this would be useful. There is nothing unrealistic about pointing a gun straight up or down - it is definitely possible, and in a real combat situation you wouldn't notice the discomfort of it.
Better idea - keep the vertical look limit at 75 degrees, but make it a "soft" limit - so if your view is beyond 75 degrees it would slowly push the view back to 75 degrees. So if you were trying to shoot straight up, you could, but to keep aiming straight up you would have to keep moving the mouse up to fight the "soft limit", which would be kind of like fighting the discomfort of aiming straight up in a real combat situation.