It's a good thought, my college professor used to push that thinking starting in Illustrator would be faster, but one thing I heavily used in Photoshop, which makes it easier doing it all in there, is layer masks.
For example, this Two-Face is something like 20-30 layers to make it. The flesh left over on the burn side is one layer, the black charring another, the mouth is about half a dozen layers for the tendons and bones etc. So even though in the image you guys see here looks like shapes, it's actually the visible elements of the layer that aren't masked, I find it easier on the blends if I can define edges with the mask then draw within that space. The black charring is made by a texture layer with several Curves Adjustment Layers applied to it for different levels of shadow and light, and then I mask them out and just "paint" the spots, works really well, but only ideal for a very monotone thing like that where there isn't much color, otherwise better to actually paint the varying shades of color in.
