Shulchan Aruch (Y.D. 283) :
You are not allowed to write one Parsha on its own, in the Torah. (except Tefillin & Mezuzos)
The Torah must be complete.
The Taz (sk2) writes the only Heter we have to write single Parshiyos, is for Children to learn (Eis Laasos Lashem).
Mori V’rabi Hagaon R. Shlomo Miler shlita said, he can’t find a Heter to write a Ketores on a Klaf. A Segulah is not enough of a reason for "Heifeiroo Torasecha" In the Altneu Shul the Shivisi on the wall, is written in Roshei Teivos for the same reason.