I haven't actually played the game. And I don't particularly agree with this - just thought it'd be an interesting dicussion.
A good team of allies can let you pull triple duty; doing a lighter version of 3 other Aspects. They can Thwart (Justice), Attack (Aggression) and block (Protection).
I presume this is balanced by the fact that Allies are expensive, you can only have 3 in play, most are unique so you only have 1 copy, and if they die to blocking then you get less than 'full value' out of them. They're also vulnerable to treachery cards too. Also their value/impact is gradual or incremental over many turns, whereas Event cards like For Justice or Swinging Web Kick give you that burst potential.
Just seems that out of the box Leadership is the easiest and most flexible Aspect to run.
Edited by jonboyjon1990