Weathering is a way of reproducing, or rather replicating the effects that the elements have on something exposed to them for a lengthy amount of time. Dry brushing is a way of controlled application with thinned paint to reproduce these effects more convicingly. The only things that I can think of that you wouldn't want to do this with would be a brand new car, or truck or other item that wouldn't have any exposure. Pin washes look fake, and too consistent when they go into every surface, and most things don't have this effect unless they've been leaking oil for some time. This oil would have to be coming from above to leak down onto everything which just doesn't look realistic especially on Sci-Fi models.
The original ILM guys understood what true weathering looked like as they would go to junk yards, and industrial scrap yards to see the effects that weather would have on them. (I remember Lorne Peterson, and Grant McCune telling about this years ago.)This is how is was done back in the day since there was no other way. It required a more tactful, and trained hand, and eye to do it convincingly. Use cotton swabs, and old T-shirts to dab, and drag oily soot, and dirt where you want it to be. Practice on scrap plastic using various shades of brown, red, orange, black, white, and gray to replicate rusty old beaten metal. A couple of mismatched panels also helps to add to the used, and beaten on look. Otherwise dry brush when you want to look like it's old, and worn not just weathered. You can use a regular toothbrush to add to this as well. Just use your eye more than anything. Remember that a little goes a long way so as not to over do it!
~ Cobra Chris