I use Photoshop, so can motion blur on demand
The dirt was never really meant to be realistic, and obviously it was just stuck on to give a more "rally" feel. I don't like to see clean rally cars
But motion blurring each and every dirt pixel sounds like a thankless task, especially when optimising it for jpg is just gonna totally screw it around anyway!
So it's not that effective when it comes to looking like dirt, but it's not too bad when you see it in motion.
Thanks for the suggestion though - I'll look into blurring if I ever try to add dirt in to another skin!