Is Dr. Pulaski unique for never coming back to any show?

She also had a scene or two in the novelization of "All Good Things...". And wasn't she a guest at the Rikers' wedding in the "Nemesis" novelization? Also a reaction shot in a DC Comic where Riker, Wesley and Selar are missing, presumed dead.

Diana Muldaur had just retired from acting when Roddenberry begged her to join TNG's Season Two. She turned him down several times before agreeing, hence she negotiated that "Special Appearance by..." credit.

She only left TNG because she asked for four episodes' leave to guest star in a reunion telemovie, "The Return of McCloud". The TNG production team got mightily annoyed because it left them to recast the CMO role for its first four episodes of Season Three - but they couldn't make her give up "McCloud" because she had no five-year contract.

And that's how Bev Crusher got written back in. It was more efficient to invite Gates McFadden back (permanently), and please her avid fans, than recast doctors again.

Later, Muldaur moved on to "LA Law" (in episodes which aired here in Oz before her TNG episodes). And then retired again. She mentioned in an interview that she never really cracked the tight family clique of TNG; the cast were still grieving for the loss of Bev, who'd been dismissed so abruptly.

It's tricky writing the return of a past character. If the actor refuses to return, the script is wasted, or has to undergo a major rewrite. And rewrites cost time and money. An actor required to return to a previous character has lots of leverage to negotiate a hefty paycheck. It's a bit different if the actor hangs around the set, or the writers' room, or conventions, dropping hints that they'd love to return (eg. Denise Crosby, Wil Wheaton, Whoopi Goldberg).