In spite of what you say there's no reason to really not run Naive Nature and the Sp. Defense drop is not very significant, whilst The Attack Drop is there whenever you attack with Diamond Storm. Just because it can KO Talonflame or other things 4X weak to Rock with a Timid Nature doesn't mean that the Attack Drop is irrelevant, since you can still very easily end up hitting something else when they switch out for less damage than you could be doing, and makes it much less spammable. On top of that, since the drop is minuscule, there is no practical special priority in the game, and it has weaknesses to Grass, Ground, Steel, and Especially Water, the scenario where the Sp. Defense boost provided would prove beneficial is virtually nonexistant in OU.