It's one of those conveniences to make it easier to keep track of space battles for the audience's sake, as opposed to "realism." Although really, the most efficient space fighter would be one that could change direction instantly rather than rely on aerodynamic dogfighting abilities, and have guns that point in all directions so that you can shoot at anything from any angle.

But then again, you can't hear anything in space either, but we hear lasers and missiles all the time.

For three dimensional space fighting, The Last Starfighter kinda takes it into account, particularly with the Death Blossom mode.

Really though it's something you don't see enough of and should be taken advantage of more. Star Trek did it occasionally from what I've seen of next gen. And heck, Borg ships are pretty much designed for it what with having no front and all...

This was brought up in Star Trek II, actually. When Kirk and Khan entered the nebula, Khan, a conqueror from Earth, was thinking 2-dimensionally out in space, so Kirk was able to confuse him by flying down under him and coming around. But other than that, sci-fi doesn't bother with dealing with three dimensions.

I guess Ender was right; what is down when it comes to the viewer?

This becomes more of a problem in sci-fi when characters are supposed to be geniuses in fleet combat and such while taking a 2-D approach to fighting the enemy. When you take the z-axis into account, it takes some of that cleverness away, especially if it's a tense battle or a no retreat situation. The simple act of flying over or under the enemy battle lines might make things less dire.

Space is flat you fools.

DAMMIT. I just left TVTropes. >= Oh, well. Can't be helped now. *browses for another three hours*

The novel Ender's Game tackled this conundrum from the perspective of infantry soldiers fighting in zero gravity.

Read the book, and understand the significance of the phrase, "The enemy's gate is down."

Sierra and Relic's "Homeworld" series of video games deals with this extensively, forcing the player to coordinate ship battles within a sphere, launching attacks often from above or below the player's fleet. That said, I think Macross uses a LOT of 2-D space.

But then again, you can't hear anything in space either, but we hear lasers and missiles all the time.

Wow, suddenly the idea of battles in space seems pretty creepy.

Doesn't that make space battles really different? Stealth takes on an entirely different dimension - since you can't hear the enemy coming, you can't rely on your ears at all. You could literally take a huge fighter or mecha that, in atmosphere, would alert you to its presence long before you could even see it because of the noise - but in space, that aspect of stealth isn't an issue, so it could be as noisy as it wanted and still be stealthier than a ninja.

And space is big - with the three-dimensional element, you have to look around in not just 360 degrees of visual motion (maybe even more to take into account enemies in trees or upper floors in buildings), but a full 720 degrees - meaning the enemy can come up behind you or from below without a hint of noise to give you any clue where they are. Fortunately, cameras placed in your vehicle could let you see all angles simultaneously, and maybe some kind of motion detection might help (although it would be utterly useless in a asteroid field, unless it was fine-tuned to take account for the motion of the asteroids).

But warning shots kind of lose their impact unless they can be seen - and if you're shooting a laser, it's invisible in space, so you can't use those. And part of the psychological effect of warning shots is the noise they make, which is moot in space.