The Makedonian phalanx -- why such depth?

[PMBardunias]

...which is to ignore the fact that Xenophon specifically says the men were drawn up in "Battle Formation" 4 deep, and that it was in this formation that they carried out their mock charge, which was sufficiently real as to terrify the on-lookers into running away!! This is not a 'parade' formation, and I very much doubt if there was such a thing! That is a modern idea in an era when drill is still performed, but has no longer any battlefield function.

They were drawn up in A battle formation and for a display (if you don't like parade). There was no single battle depth in any case. You know I believe he specificaaly calls it a "battle formation' because he was most definitely NOT in a battle at the time. The other incidence is of troops displaying their drill on the way to dinner!

What purpose do you think that Xenophon's 4/6 deep formation serves then?

What do you do if you are in close order at 8 ranks and being outflanked? Nice to have an option. Also, you can countermarch off the rear half of a unit leaving the front 4 or 6 ranks.

If it is merely to form a thinner than usual line, why is there no 2 deep formation?

Why do you think there wasn't? Isocrates mentions Spartans in a single line.

We don't have much evidence, but what we have points to 4/6 deep in close order as being "battle formation", formed close to the enemy from "normal" formation in open order 8/12 deep. What grounds do you have for not accepting this evidence?

First I find one instance where a "battle formation" is mentioned in the context of what is clearly not a battle to be far too little evidence to hang my hat on. Second I don't believe that the greeks would stress the depth of the penultimate number of ranks. We have no evidence of early 5th c hoplites and hoplites not versed in the spartan drill system doing anything by lining up and charging. Your reliance on bringing up the rear half of columns ignores that this is barely mentioned in Aelian and Arrian while the movement of alternate ranks is stressed. The movement of whole groups as well is given attention as well, so opening ranks for light troops need not be done by files waiting to double in opened order.

When we look at the bigger picture, this hypothesis solves all the anomalies of depth in our ancient literature.

Perhaps this is the problem. I see no reason that there be the same standard for all of the time periods we are considering. It would not suprise me if for instance the terminology changed between Classical and Hellenistic phalanxes. I think this is clearly so with spacing, since as I mentioned synaspismos occurs at anything less than about a meter for hoplites, not 0.5 m.