The BG do pass on thier other memories from one Revernd Mother to anouther at the time of death. However, the memories that get transfered are only memories accumulated by the BG. This insures that no details are lost. Every Reverend Mother already has direct line other memories from their own ancestors. But since not every RM is related biologicaly, they must share and pass on other memory between each other. Some of(or all) of a RM offspring will never become RM's. This means that, if the BG want to keep information within other memories with them at all times, they must pass it on before death. Sure, RM's can come from later generations from a current RM offspring and then gain the knowlege(skipping generations from one RM to the next). But this doesn't allow the info to be on hand when they need it and potentially it could be lost forever if they don't get it before a RM's death.
