we really need to define this; when most think of realism in gaming, theyre really thinking of immersion. realism contributes to immersion, but it isnt intergral, nor is it all that you need. the general idea is that the more 'realistic' the game the more believable it is and the easier it is to get into because of it.
gamers and developers alike often fall prey to the assumption that realistic=immersive, or in developer's case at least try to market games as such. i could go on for days about what makes an immersive game, but the key point ill make is that the contribution realism makes to immersion is all about being believable rather than actually realistic; for things to move in way you'd expect, look more lifelike, but for the most part little beyond that. video games are escapism, and realism is only there to make the fantasy aspect easier to get into, not to overtake it.
that, and real silenced pistols dont sound nearly as cool as the james bond sound effect we all know and love.