Even in this case, some software which is going to send a packet to the network
from CPE, according to RFC3484 "Default Address Selection for IPv6",
there is a certain reason why the WAN I/F's IP address should be used as
its source address like follows.
---------------- from RFC3484 ----------------
It is RECOMMENDED that the candidate source addresses be the set of
unicast addresses assigned to the interface that will be used to send
to the destination. (The "outgoing" interface.) On routers, the
candidate set MAY include unicast addresses assigned to any interface
that forwards packets, subject to the restrictions described below.