That's because in some airplanes you could sit up higher and see over the nose and in others you couldn't IRL. Since some planes had this innate advantage while others didn't it should be modeled in the game to maintain the various plane's relative advantages and disadvantages. For example, you can see way over the extremely short nose of a 110 but you cannot see over the extremely long nose of an F4U, no matter how high you sit - this has been modeled and rightly so.
Except in reality, if you adjust your seat up high, you'd also adjust the gunsight reticle to be centered accordingly. Effectively, in the virtual world of AH every pilot has already 'adjusted' his seat - which is where the center of focus is set as in the default screen.
Thus, moving the head position up is literally "moving the head up". Conceptually it has nothing to do with individual advantages/disadvantages between the planes in how far one can see over the nose, but rather how much headroom is available in the cockpit, and how much neck/back/torso movement is allowed for the virutal pilot in the cockpit. In the older version planes these restrictions were very lenient, so far as to in some cases the pilot's cranium might be sticking above the cockpit glass. In the more recent game versions and plane models, the restrictions are far more strict.
Because a flight sim is an imperfect environment (especially related to views) there are some less-realistic things that sometimes are done to actually help maintain the balance of the plane set. I say set that cursor and take advantage of your over-the-nose view to capture the advantage that the real pilot had.
The comfy cozy computer room chair, and the thumb-controlled hat switch flippin' (or, in recent cases neck wriggling with Track/ir) to move views around at physically impossible/improbable situations devoid of human anatomical constraints or G-force constraints is already overcompensates for any 'imperfect environment' in the first place.
Also, the so called 'real-life advantage' as mentioned would hardly - if ever - have actually taken place in real life. A pilot may squirm around to peer around the nose for a glimpse (that is, if only the plane isn't already generating enough G-forces during maneuvering) of the enemy, but you can bet your money on that he will return to his normal seated position when he is ready to pull the trigger.
What I am trying to say, is that it all balances out and is less gamey than it seems, imho.
No it isn't. It's a gamey thing which has nothing to do with 'reality issues' at all.
It's allowed in the game as long as AH doesn't go so far as to implement some kind of "pilot action physics" system which might mimic realistic constraints of pilot movement under stress of real life forces and pressures according to plane maneuvering. However, hypothetically speaking, if AH ever implements such a system then you can bet the
'peering over the nose and spontaneously aiming at the target with an uncentered reticle view, while pulling a turn so hard enough to necessiate such action in the first place' definately won't be seen in the game.
Until then, people may use it freely - but that don't change the fact that it's pure gaming the game.