I've intimated this in quiet ways over the last while, but I might as well go on record as saying that I think the current trend of painting uv-mapped wear and tear onto everything is an enormous waste of time. I've tried various methods of making the process more automatic, but it's all pretty crude.
On a whim, I decided to visit the site for Modo, and lo and behold, these folks now have what they call an occlusion layer. [link]
Yes, yes, I know that we've all had ambient occlusion for ages, and I also know that it's possible for different programs to bake an AO solution to a texture. What's cool about the Modo solution is that a few clicks can customize their occlusion solver for various looks (like weathering on pits and/ or bumps, top-down weathering, etc.) You can randomize things, bake the solution to a texture, use that texture as a mask for painting rust and dirt, and all that good stuff.
Of course, with this development, the CG and games industry will now decide that "dirt = real was SO first decade" and make everything look pristine...
If I'm not officially on record as having said it, here it is:
Dirt and damage on 3d models ought to be a procedural thing.
However - please note that all of you UV-map painting wizards who can do this by hand and make it look good still impress me.
Now...I wonder how I can replicate some of this functionality in Blender...