I detest ADS system, even merely a visual one. I do not find them cool. I find them claustrophobic, obstructive. Aesthetically, pure hip-fire is better, i can see my gun AND more of environment. Seeing muzzle-flash light my gun looks far cooler than ADS, and the same muzzle-flash is not obstructing my view either with hip-fire gun. If immersion and realism were important for a game, i'd accept ADS... if i played such games. Halo isn't even aiming for realism.
The classic scope view in older Halos is clear (the "black" is transparent, gives you quite good peripheral vision). It is also faster to use (no animation, which is better).
Gameplay-wise, i detest ADS systems in most games. I need to slow down my movement and obstruct my view to be able to hit something. How boring. How annoying.
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And it has perfect lore explanation as well, the scope is a camera that is linked to the users HUD (Spartan's helmet, Marine's glasses or eye-screen). In Halo-verse, there is no need to aim the same way you do with modern, real weapons. (No comment whether it is actually better, easier to shoot gun without having to peer through iron sights assuming you somehow know where you're going to hit.)
There is also the pointlessness of the change to consider. If it has no effect, unlike most games ADS system, why even have it? Isn't the old system good enough? I think the change exist only to pander modern shooter fans, in other words, it is a slap into face of older Halo fans who like Halo as it was, old-school with no ADS (there is more to Halo of course, but we're discussing ADS ONLY here). It is pointless as pandering, those who like ADS are not going to like ADS in Halo if it doesn't work like ADS usually does (unless it is the visual effect, but then we run against the lore part, and i do care about lore-gameplay connection whenever it makes sense though gameplay naturally trumps lore).
And if it has effect, it is very un-Halo, goes against the simplicity of run-and-gun which Halo embodies (despite its somewhat slow pace compared to, say, Quake).
Halo has uniqueness that derives from lack of ADS, in addition to its many other qualities. Sacrificing these makes it a not-Halo, why should i care about a not-Halo as a Halo fan?
And its effect on the control scheme. The perfect place of grenades in left trigger is given over to zoom, zoom that has always been situational in Halos, not something you use everywhere unlike grenades. Perhaps there are alternate control schemes but the default control scheme should be as close to the old controls as possible for ease of transition (if you play with defaults).
I do not oppose changes, when they're sensible, or add something to the game (Halo 3 equipment added something to the game. The change to Armor Abilities in Reach also added something to the game). But when the change exists for merely change's sake... it is questionable venture. If it were a graphical overhaul, i wouldn't care so much. But stuff that affects screen clarity has effect on gameplay as well.
Is this enough? I really have reasons for my opinions even though it seems you want to assume otherwise.