I wondered how dangerous Quachil Uttaus actually was during the game, since they'd seemed to be one of the easier ones outside final combat. The first player will survive 7 turns but not 8 half the time, if they don't spend clues. The mean survival time - since there's no upper limit - is going to be higher than the median. In perhaps 1 in 100 average length (20-25 turn) games, no-one gets devoured at all, in fact. More likely, there'll be two or three Investigators devoured by the Dust deck.
With that being the case, it doesn't usually seem worth spending clues to prolong an Investigators life, unless you're on the third deck already and absolutely need them alive just one more turn - it probably won't change the total number of Investigators devoured during the game.