My non-scientific opinion: Once actual combat is engaged, then players are already re-oriented on a new plane instead of the default plane of the straight ground - the biggest advantage is that initial time cost of each player creating/aligning that new plane. A higher up player notices the lower player first, fully aligns his plane, then shoots. The lower player, while being shot at, still has to take time to notice the upper player, and then re-orient his plane of combat, giving the upper player more opportunity to do damage before being responded to.
A few other points to consider:
- Higher ground lets you see slightly more behind barricades, which might be enough to shoot the enemy hiding behind it.
- Grenades travel farther from above, since (being already elevated) it takes longer for them to reach the ground.
- Depending on the nature of the level, the upper player can just back up (pulling himself out of view) to get more cover. The lower player must actually find cover.