Comment by wildzzz

It could just be that using Ethernet allows for more flexibility in what is connected to that board-to-board connector. In this instance, it's a wifi router but a customer may instead want a bare port to connect to their existing infrastructure (just the modem). It also means you don't have to worry about which MII standard both boards use, they can be upgraded independent of each other. The physical interface for Ethernet remains the same whereas MII has different pin counts.