Board Thread:General Discussion/@comment-24815992-20140516131902/@comment-8986886-20140516204706

"Pedantry and mastery are opposite attitudes toward rules. To apply a rule to the letter, rigidly, unquestioningly, in cases where it fits and in cases where it does not fit, is pedantry... To apply a rule with natural ease, with judgment, noticing the cases where it fits, and without ever letting the words of the rule obscure the purpose of the action or the opportunities of the situation, is mastery."

—George Pólya

Or, to put it differently: When Yammy released his Resurrección, he was basically the last Arrancar fighting force left within Las Noches. When the alternative had been to surrender Las Noches to the enemy, then breaking a rule or two, to increase his chances of winning, seems justified to me. I doubt that Aizen would've reprimanded Yammy for his decision.