The fact that all the necessary retail labeling was printed on a card slipped into a transparent pocket on the front was nice, because it permitted a much classier treatment of the paperback cover itself - plain red with black lettering.
Remove the cover insert and you remove all mention of the title "Star Trek", making it look more like an authentic Starfleet manual.
When the blue "Starfleet Medical Reference Manual" came out, Ballantine probably planned the same vinyl cover treatment, as that book came with a separate cover insert (again, the only "Star Trek" reference on the work). I bought a spare "Technical Manual" (and sold the red trade paperback book at a book exchange) and slipped my "Starfleet Medical Reference Manual" cover insert into the front of its cover. Matched set!