In my particular case (i LOVE doing post work) i began to use them, like a LOT. Every render pass has it's own benefits if you begin setting them up above your main render layer. The diffuse pass lets you recover any color data lost or atenuated with lighting by boosting the original colours you've selected. Reflection and AO passes are a must to bump those shadows/reflections. You just have to play with the layer blending mode in PS and then you adjust the layer's opacity to set how much do you want to affect the render. All this lets you tweak and enhance your image in a nearly 100% non destructible method and making changes easily. It takes a bit more time, yes, but it's worth it imo.