It probably depends on the modem. For example on the old Speedstream modems, it was best to offload the PPPoE work to the router as long as the router had a decent CPU to handle the load. This allowed the modem to handle more connections (sessions) which was needed for certain online games.
There are various ways to set up a modem and router to allow continuous access to the stats. These methods can probably be adapted to other modems and LAN subnets: »
SBC DSL FAQ »
How do I access the modem GUI thru a router - advanced methods?