Why specify a default router?

[The Orange Rider <theorangerider@gmail.com>]

Hey all, I'm just entering the IPv6 world and I have a question I'm
hoping someone can answer:

Time and time again I read about how hosts don't need to have a
default gateway configured statically in IPv6 as they did in IPv4
becuase they will automatically discover the router via Neighbor
Discovery Protocol (through Router Solicitations and Router
Advertisments and all that).

This is all well and good, except that Linux still supports the manual
configuration of a router (route add -inet6 [ipv6_address]). What I 
don't understand is why this is supported at all. Since IPv6 lacks ARP
requests, it seems as though a host would still have to resolve the
router via NDP even if it is manually configured with a default
router. Can anyone clarify how specifying the L3 address of the
gateway without an L2 address along with it is useful to the host?

It's been suggested to me that this may be for security reasons to
make sure that the host isn't using a router that has snuck into the
network, or that it might be used to force a host to choose a certain
path when presented with equally efficient options on which to send a
packet, but I thought I'd check what the IPv6 experts have to say.

Thanks a bunch!