The weak model doesn't go into such detail, it is assumption/impl. detail
to assume that the ip address still is part of the system even when the interface
is down. One could just as well define interface down as temporarly removing
the IP address from the system too. This makes make much more sense to me and
if you always want the system to answer on a IP adress you make it an IP alias.

Since the current behaviour is a problem to me and routers in general, can
we change this? What is the current usage model which needs it to stay as is?

Router != end-system  so I wouldn't think the weak or strong end-system model 
would apply to a router.  I think Stephen already posted a patch to allow that 
for when one's box was a router rather than an end-system.

rick jones
It's a router!
No, it's an end-system!
...
http://snltranscripts.jt.org/75/75ishimmer.phtml