Sometimes "not seeing" is better than "seeing". Not seeing the Krell leaves them as mysterious beings. There are many movies where "not seeing" is in some ways the key to the movie. In "Alien", the alien creature is almost never shown except for a few seconds (Ridley Scott was afraid the actor-in-a-costume would become obvious). In "Jaws" the shark made few appearances (due, apparently to technical difficulties), but that worked to the movie's advantage.
Very true, but sadly, most films employing that technique by design, or by default in an effort to salvage "something", fail miserably.