Agreed - PTC doesn't help much. If the entire part is one color/texture then the decal works OK, but there isn't a facility to simply overlay appearances - I don't believe one can have separate bump, color texture, and decals applied to a single surface; instead PTC makes the user combine them explicitly and apply the combination.
Suppose there is a part with three colors and one bump map. If the bump map changes there are going to be three combinations to change. If a surface gets a decal, then that's a fourth combination. If not all the surfaces get a bump map you could have 2 x 3 x 2 =12 appearances to manage, based on 5 items (1 bump, 3 colors, 1 decal)
If you get the chance, check out POV-Ray. The textures for it are procedural, and they have image maps. Very nicely done and easy to produce beautiful, realistic materials with relatively simple descriptions.