What's the point? Route everything that comes into the router over PPPoE and than you don't need any filters.
The point is so that nothing comes into or out of my router except PPPoE traffic. No MNDP/LLDP/CDP, no STP, no ARP, no OSPF, no RIP... need I go on?
My ISP is an all-bridged WISP, I plugged my "WAN-side" interface into their CPE so that interface is part of the whole WISP's L2. I know it's their responsibility to secure their network, but I can do my part by not accidentally flooding their entire L2 with STP PRIO=0.
Also besides my use-case, an ISP using MT as their PPPOE Access Concentrator (AC) would have that AC on the same L2 as all their customers. They would want to ensure customers can only reach that AC with PPPoE and not try to port-scan, ping-flood, or try to do DoS the AC using MNDP/LLDP/CDP or STP.