This question was brought up on the Tripwire Interactive forums with regards to Red Orchestra: Ostfront and the upcoming Heroes of Stalingrad; a thread requesting a field of view decrease while using ironsights evolved into heated debate, with both sides bringing up interesting points. For the most part, the dispute was more concerned with the fact that either atmospheric or gameplay realism could be achieved, but not both at once - it was taken as agreed from early on that decreasing the player's field of view to create a zooming effect was the closest possible method of replicating real-life perceptions of object scale when aiming through ironsights.
When you're faced with two mutually exclusive options like this, what do you do? Red Orchestra refrained from introducing sight zoom, turning even relatively short-ranged engagements into a nightmarish game of 'pixel-hunting'. Beyond a hundred yards, even the largest monitors at the highest resolutions would fail to effectively tell the player what they were aiming at, and whether it was likely to shoot back. Some would argue that visually, this was more realistic or immersive (personally I barely notice the change in games which use it sensibly), but there is little doubt that it made long-range shooting or even target identification hellishly difficult, especially in a game where one stray bullet or random artillery shell could mean a five-minute walk back to the front, sometimes far more. With this taken into consideration, I would be far more inclined to side with the gameplay camp. But what if it was something drastic, something which is very noticeable? What do you do when you want to make an immersive and realistic game, but you have to choose between the two?
Naturally, the answer would differ from game to game, with the end goal being a precarious balancing act. If the game's a simulator (ArmA), obviously realism is the concern. If it's a shooter, though, I would say that atmosphere would usually be the primary concern. If a game feels right, then it will be rewarding even if some aspects are not entirely realistic. Despite the necessity of going pixel-hunting in Red Orchestra, it remains one of my favourite games because it certainly does feel right to me. At the end of the day, that's what brings me back.
Thanks to Matty Dienhoff for taking this matter up with me earlier today, and linking me to the relevant thread.