There are some sitcoms that simply don't work if you're down so much as a single cast member. Could you imagine an episode of "Cheers" without Sam, Carla, Cliff and Norm? The series' producers and writers couldn't, which is why they never missed an episode. The same was true on a much larger scale for "The Brady Bunch," where every single member of the family (and their live-in housekeeper Alice) reported for duty on all 117 episodes.
This kind of consistency was evidently key to a successful Sherwood Schwartz sitcom. The writer/producer who created "The Brady Bunch" was also the mastermind behind "Gilligan's Island," the CBS joker about seven castaways who wash up on the shore of an uncharted island somewhere in the Pacific Ocean. The key to the show's success was its shamelessness: from the pilot to the series finale (which arrived a little sooner than expected thanks to "Gunsmoke"), you could count on seeing every member of the ensemble engage in all manner of inanity (just like you could rely on them remaining on that island, no matter how hard they tried to escape, by the time the end credits rolled).
Schwartz wasn't one to deviate from formula, but he was wise enough to know early on in the show's run when an actor he'd cast wasn't clicking with their co-stars. How early? Try the pilot, which is the only episode of "Gilligan's Island" that didn't feature three of the show's most beloved actors.