I've been doing digital art for quite some time now but I'm still uncertain about this. I think there's a benefit to both. If I use 1 multiply layer, I can finish illustrations faster and I can have a more consistent contrast of shadow. However, sometimes I want the clothes to have a different contrast of shadow to the skin. That's where multiple multiply layers for shading is useful. Also, in my opinion, you get to use multiple colors as the shadow when you use multiple multiply layers to give your illustrations more variety in shade of colors.

Please correct me or add lacking information to my statements above. There could be some inaccurate information there, they're just my opinion based on my personal observation.