The explanation I got for the rationale behind the rapid rise came, from of all places, the instructor at STU (Sram Technical University.)when I attended their classes about a dozen years ago. As he explained it, the move to 9 speed wide range cassettes made the angle of derailleur movement slightly miss matched to the steepness of the cassette face. The rapid rise was Shimano's answer to the problem, as now the spring would move the derailleur to the larger cog rather than positive motion, and the potential for binding was reduced or eliminated.
FWIW.