My analyses were based on average damage taken, to be clear - it is possible guard is worth doing in specific circumstances if taking standard deviations into account. And you might be underestimating just how strong "just try twice" is - if the chance is 50%, 'guard' will give you a 60% chance, but trying twice is a 75% chance. Plus, you have a chance to just not take any damage, and in a game with this many encounters attrition really matters.