I can agree with using pocket screws instead of biscuits any time, I see them as about equal in strength and with the pocket screws there's no need for clamps. In my opinion a well fitted mortise and tenon is somewhat stronger than screws and glue so still has it's place but not likely in a simple bookcase. Actually what I had in mind for the bookcase in question was rabbits and dadoes. A shelf set in a groove is actually supported by the sides rather than screws and/or glue, I know with modern glues this is less of a concern but I still feel that any joint that will hold itself together without glue or fasteners will do so even better with them. But , it was never my intention to discount the usefulness of pocket hole joints, I use them often and, unlike biscuit joints, cannot recall a case where a pocket hole joint has failed me. I am a big fan of my Kreg jigs.