I know only the advantages of Layered design. We can treat each layer as a problem area. Say for example, data link layer, people can concentrate only on the device driver only, they dont need to worry about the network related meaning how the packet can be routed etc. The only responsiblility of the device driver layer is to grab the packet destined to the local MAC and pass the frame to the network layer. Similary the network layer doesnt need to worry about how the packet is reached to the local machine. Its only responsibility is to check whether the packet belongs to the local machine or to be forwarded. Also build the routing table to forward the packet. This way, each forum can precisely concentrate on one problem/layer.