Now this is one of the odd features that Im actually interested in. There's nothing unrealistic about it, provided its a modest implementation. In theroy when you play RO, you are still essentially controlling an avatar, as the weapon sway, stamina, etc is all native to your character; and as such, you have to fight with those impedances.
It makes sense (at least to me) that if you have played RO2 for years, that you should get something to show for it, such as better overall morale, slightly reduced weapon sway, marginally faster bolting and reloading.
My only worry would be that it's too easy to "level up" and as such sooner or later everybody will be running around with their gruff neck-beards and war medals with no rookies in sight. I think it should literally take you months and months, if not years of playing frequently before you get those distinguishing attributes and earned innate skills, so seeing an "RO:2 veteran" would actually be a novel sight instead of just an average experience... People who play the game frequently for a long period of time should get those distinguishments.
Rather than a short-term gimmick like perks in call of duty, it should be a long term achievement that you earn after a very long period of time.