RFC 2462 specifies:
5.5.2. Absence of Router Advertisements
If a link has no routers, a host MUST attempt to use stateful autoconfiguration to obtain addresses and other configuration information. An implementation MAY provide a way to disable the invocation of stateful autoconfiguration in this case, but the default SHOULD be enabled. From the perspective of autoconfiguration, a link has no routers if no Router Advertisements are received after having sent a small number of Router Solicitations as described in [DISCOVERY].
If I understand correctly is that if a host fails to receive a router advertisment upon solicitation or within specified time limit, it should use DHCPv6 for address configuration.
Since DHCPv6 cannot carry the information about default gateways (http://tools.ietf.org/html/draft-droms-dhc-dhcpv6-default-router-00 is deprecated), what is the point of address configuration (the address cannot even be used for link-local traffic since DHCPv6 lacks prefix information)?