Sure, it's great to leave things to the imagination, but the imagination needs to have a little something to work with. An effective, horrifying image such as this might even have inspired the writers to come up with better material. Their imaginations need something to spark them, too. Asthma and a locked door over and over wouldn't inspire me, I know.
Given that their other attempts at F/X were so horrid--I speak here of the Moon Poppy, the Construction Paper Shadow from Hell, the Incredible Shrinking Werewolf--I tend to think that not depicting the Leviathans was a very smart move on someone's part.
There's a scene in
The Bad and the Beautiful that was based in part on Val Lewton's experience making
Cat People. Small budget film and the intent was to have actors wearing a jungle cat suit to depict the monsters. In the movie, we see that what ends up happening is that it looks just like actors wearing jungle cat suits. So the producer decides never to show the audience exactly what the jungle cat looks like. 100% more effective.
They would have had some stunt guy underneath a painted sheet or something. His hand would have been visible or his face and they wouldn't have stopped shooting. And it would have looked exactly like some stunt guy underneath a painted sheet.
