It depends what you want to do with the Ethernet port.
When you use the AE as a Router, then the Ethernet port becomes the WAN port (i.e. connect it to a Modem or another Router - the E port is isolated from the WiFi "port" by the AE's NAT).
If you just want to bridge networks and just connect the AEs WiFi to another network (no NAT operating), then you can connect a switch or a Router to the AE's E port.
This restriction is probably why the new versions have 2 E ports.