Its "nearly" impossible to reconstruct your render result using the render passes only. To get anything close requires a lot of work and makes your composit very complex. Its important to be to reconstruct your render result when you need to change only 1 thing, without changing everything else.

Example:
If you just need to change the color of a shadow in your render you can use the shadow pass and make the nescecary changes. Now getting the image back together with the changes made for the shadow pass is a nightmare. You can end up with the specularity in your scene looking different, even if thats not what you want.

There have been discussions about how Blender's compositor is meant to "enhance your renders" and not recreate them. This is of cource fine if thats al you need to do. If you need more relyability and control you should be able to "put back together" what you take apart.

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