Swords Dance / 3 Attacks is useful if you're willing to take a strike from the opponent and hit them back hard. For example, you may not know if the opponent's Tank is a Hippowdon, Ferrothorn, Skarmory, Togekiss, or whatever. Or maybe, they'll decide to switch in a "check" against you, like TTar (who'll resist Shadow Ball / Shadow Sneak / Shadow Claw). So instead, you use Swords Dance and hope that the opponent switches in a pokemon that Aegislash Shield Forme can survive 1 hit from. Swords Dance + True attack probably will hurt the opponent more, and leaves you in safer shield form for the switched-in attack.
Case 1
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Turn 1: You know the opponent will switch out for some reason. Aegislash uses Sacred Sword, predicting a TTar Switchin.
Opponent actually switches in Togekiss. Togekiss does LOL on the Sacred Sword. If you chose Iron Head, then assume the opponent switched in Ferrothorn instead. Its not like you actually know what the opponent's wall is yet.
Turn 2: You're in Sword Forme, and are forced to switch out. (Shadow Ball Togekiss wins in Sword Forme, and Sacred Sword + Iron Head isn't enough to win vs Togekiss)
Case 2
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Turn 1: Same situation, but you now "know you don't know" what the opponent will switch in. You stay in Shield Forme and just Swords Dance.
Turn 2: Now you know your opponent's switch in. +2 Iron Head OHKOs Togekiss. +2 Sacred Sword OHKOs Ferrothorn or TTar. If you discovered your opponent's Hippowdon, Hydregeon, or other pokemon that wins... you switch out.
Basically, use the Swords Dance as a crutch. Why rely on prediction, when you can just "save up this turn's attack" for "next turn". Bonus points because you remain in Shield Forme while Swords Dancing.
Of course, this only works when Aegislash has high HP. And the strategy is still beaten if the opponent plays an aggressive switch-in like Heatran. But if you feel like you can predict "some sort of wall" coming in, and you need that perfect prediction for 2HKO... why not just swords dance on the turn while the opponent switches out?
There's no need to get to +6. One turn of Swords Dance is still quite useful. Relying on +6 shennanigans will get you utterly owned if your opponent is anywhere close to half-decent.
alexwolf EDIT: Removed the quoted part as this post has been removed.