Well, I would not go so far to say these types of tracks would be less dirty. The mortar tracks were right up there w/ the main body and in one good example, at Objective Curly during the second thunder run of OIF1 they were right in the middle of it all, sitting on the road putting rounds down range (not that far down range though).
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Even rear echelon vehicle types get quite dirty. Just take a look at the HEMTTs and other support vehicles. They go through much of the same ground as the front line vehicles. Also, the dust clouds in those convoys are something to behold as I have witnessed at the NTC.
Just driving around in a desert environment gets them dirty.
Weathering is more a matter of taste. Either end of the spectrum can be correct. In OIF1 there was rain early in the campaign, so there are even opportunities for mud on vehicles. As both the Army and Marines got closer to Baghdad they stayed on roads moreofte, but not STRICTLY, so even in the later period of OIF1 the vehicles were plenty dirty.
I have spoken to Gino about his weathering before, and he likes to take the equally correct stance of not so much mud and dirt, etc, since he likes to show off the vehicle itself (sorry Gino if I did not quite word your argument so well), and many of his builds are desk models for customers.
Many times I do the same especially when I have done some detailing which I do not want to cover up.
I for one do not think modelers overdo the weathering (maybe the chipping though - at times) as it looks cool, and many of us like to show vehicles and machines in their toughest environments, which includes dirt and mud. I think besides the "things go boom" cool factor, we like AFV modeling because these machines are TOUGH. What better way to show that than weathering the thing?
Finally, be cognizant of the photo resolution when looking at whether a vehicle is clean or not. If you study most photos you will see at least subtle weathering all around a vehicle. At first glance the attached mortar track is not really dirty, but when you really look at it you can see the obvious dirty back ramp, but even the rain streaked fuel tank is evident and the tracks have dust in them as does the wire coil. If you were able to zoom in on this photo like I can you can see rain streaks all down the sides of this vehicle, and even some grey numbers in the Velcro square.