With PPPoE, the client (your router) has to communicate with (and authenticate with) the ISP's server. Once that works, you have an IP address and can do stuff. If you put an ATA in front of router, then the ATA needs to do the PPPoE authentication with the ISP, since your original router is sitting behind it and can't do PPPoE. Not to mention having to deal with possible double-NAT issues from apps on my internal LAN.