The way I see it. A vignette, diorama or just plain old model is done by the modeller as a 'snapshot' in history, like someone has jumped into a scene, clicked the shutter and jumped back. This could be mid battle, before, during or just after crossing wild cold wet muddy terrain and the crew taking a breather before they start looking to maintenance etc, also, the vast majority of tanks modelled on forums tend to be 'in war' examples. The only heavily rusted model tanks I've seen are whif/post apocalyptic or abandoned during a hasty retreat, things like exhausts would rust more readily than others and again in wartime would not be high on the list of important tasks, 'Hans, ven ve reach ze next village be a dear a pop to halfords for some of zat high temp paint vud you' Also, the other side of this is, if you like the model and it's your model and you haven't built it for a customer or competition with strict requirements, who cares? The beauty of tanks and AFV's I think is that it allows people to get dirty, instead of the pristine parade ground examples anyone can build out of the box and while I've only built one tank, (2 if you count the tiny Zvezda thing) I love to see creatively weathered AFV's realistic or not. Sure we can all be picky about things, I'm one of those who'll watch a movie and go, '10 shots from a 1911 without reloading? Really? But then I remember to shrug and enjoy the movie anyway.