To hear the switch vendors tell it, this is an easy move. But don't be fooled.
Customers typically organize their routed networks into separate VLANs.
Traffic from the different networks is then identified across interswitch links through 802.1q VLAN tagging. In this configuration, the router at the edge of the network is required to forward traffic between these VLANs, and there are really only two ways to do it. However, this requires that the router support 802.1q - something few traditional routers do. In the second approach, the router could simply transfer traffic between different physical interfaces, one per network. This follows the traditional multiport router model.