To be fair - when he did "Unification" at the time it WAS a marketing tie in to the release of "Star Trek VI: The Undiscovered Country" (released generally the same week that his appearance aired - and the film's events were minorly referenced in the episode itself.)<--- So there may have been a bit more of 'self interest' to see his film do well at the Box office; when he agreed to appear on TNG for "Unification II" <--- Which because of Nimoy's schedule was actually filmed prior to "Unification I".