The fastest commonly available internet connections tend to be around 1.5Mbps, or Megabits per second, which is one-fifth of the speed 10BASE-T (10Mbps) is capable of achieving. The fastest cable and DSL internet connections that we have seen advertised claim burst speeds of up to about 6Mbps, which is still considerably slower than 10BASE-T Ethernet.
The second reason why we do not include 100BASE-T capability in the WAN port is that the XRouter Pro has a maximum throughput of about 8Mbps (as tested on a LAN using FTP). This means that even if you had the ability to connect it to a 100BASE-T internet connection, you would still only be able to route data to your local network at a maximum speed of about 8Mbps.
Using a 100BASE-T Ethernet WAN port on the XRouter Pro would raise the cost of the unit for no reason.