Hmm - you seem to be assuming that the only reason for router A to send an RS to router B (and get an RA back) is for the purpose of autoconfiguration (i.e., RFC 2462, section 5.5.3). I'm not assuming that at all.
I believe it is useful in some cases for router A to discover the prefixes being advertised by router B w/o necessarily seeking to autoconfigure addresses based on those prefixes. (RFC 2461, section 6.2.7 gives one such example.)
i guess i mentioned it in the past, if we allow routers to advertise different set of prefixes (which is the case now with RFC2462) and allow routers to autoconfigure themselves with RA, we go into bootstraap problem when router reboots. i consider autoconfiguring router is a dangerous idea, and "hybrid router" idea is too.
itojun