Why study the provisional teachings?

"Likewise, even after the provisional teachings have been opened up and merged with the Lotus Sutra, one must not accept these evil doctrines, which have been rejected and cast aside as mere “rough teachings,” or intone their names or words or the principles that underlie them, mixing such doctrines with those of the Lotus Sutra." -- Nichiren

We study  them  in order that we will avoid mixing them with the teachings of the Lotus Sutra. Because SGI leaders are unlearned they think nothing of mixing and confusing such noxious doctrines as Yui Yoga [mentor and disciple] with the teachings  of the Lotus Sutra.