No offense, but the "[insert random character] should be used more often" argument is too vague and can be applied to practically every character out there.

Why exactly should DC prioritize Scarecrow over every other Batman villain who isn't the Joker? You say that Scarecrow "really can be written amazingly, and can be a three-dimensional villain with interesting goals and actions who poses as a genuine threat to Batman without being stripped of all his depth in favor of his powers.", but that can be applied to any other Batman villain, like Bane, Two-Face or Poison Ivy.

Also, the term "definitive" is thrown around so much that, quite frankly, it doesn't mean anything anymore. "Definite" means "conclusive, reliable, with authority", which means that creators should use a particular aspect of a character, like Scarecrow, instead of experimenting every once in a while to avoid repetition. If you say that a specific appearance of Scarecrow is "definitive", then you're somehow implying that other appearances of Scarecrow are fakes or imposters that can never measure up to the genuine article, no matter how hard they try.