You can say the free cast thing about destro, but ultimately it never works. WotLK proved that. Hell I think the day Ulduar went live that conflag was fine before they nerfed it into oblivion "What do you mean this spell that can only be cast when immolate is up and destro has no dispel protection actually hits hard??!?" There were literally rogues and one other class saying it was bs while they wore only 1-2 resilience pieces citing "I am a rogue I shouldn't have to wear resilience"
No matter how easy it is to shut down destro, Blizzard will cave if people(bad players) complain.