The Makedonian phalanx -- why such depth?

[PMBardunias]

2. We are NEVER told that Hoplites actually fought "8 deep" or "12 deep" etc - Xenophon speaks of 'battle formation' 4 deep, or of Spartans nominally 12 deep as "6 deep"....

To be fair, both the 4 and the 6 rank depth occur in the context of parade, not battle. I have no problem with their drill being able to go to 4 or 6, I simply don't think it means they regularly did. On the principle of "this one goes to 11", it would seem useful to be able to double one more time than commonly needed. We don't really know how tight the linkage between doubling by half-file and spacing is. Aelian makes clear that men are expected to change their spacing by opening and closing in the same number of ranks. Notice we are wisely not addressing what this means for the line as a whole. Eventually we'll have to reconcile Aelian and reality.

I think we have more evidence from actual battle (Nemea, Leuktra) of whole units counter-marching behind others to deepen or extend the line than of doubling by rank. The reverse would be a unit closing to the right by simply stepping laterally, followed by the rear half of the unit moving to fill the new-formed space in close order.