Given the requirements and pressures of TV acting at that time, LN might even have been worried that he wouldn't be properly credited (and paid!) in that episode.
Onscreen credits and salaries are established when contracts are signed. He had no fears there, but this turned out to be the premiere episode of the season, not a good one to be almost-totally absent for, although airing order wouldn't have been known at the time).
Playing a zombie may or may not be an acting challenge (it's probably way harder than one might imagine), but at least it puts him in the middle of the camaraderie and creativity of the rest of the shoot. Leaving Spock unconscious in sickbay for most of the week requires minimal participation, and four days away from Shatner's ummmm, screen-hogging.