My interior is spotless. I detest clutter in my house, moreso in my car interior. The outside of the car, I'm not so particular about, don't get me wrong, I have the thing washed very few weeks. But a splash of mud here and there just looks right.
Like andybaggies said, it shows the thing's been driven. My car is in good condition, but cosmetically it is not perfect. And it is an old car by this country's standards. I think I'll keep the mud for a while, like a pretentious badge of honour.