The Makedonian phalanx -- why such depth?

[Paralus]

.....This is something of an assumption on your part, he does not omit the dimoirites/half-file leader,

I do not have the Greek but, as the file leader is named as such, if the word dimoirites was used by Arrian it would be translated as such - as file leader was. that it wasn't indicates Arrian did not use the term.

I assume nothing in fact, simply state what Arrian has said rather than assume - as you - what he might have meant. You assume that these descriptions mean "corporals" and other NCOs yet Arrian nowhere states such in his text. It is self eveident, based on Arrian's description, that this phalanx was to be used in a simple straight line of attack. The weight of attack will be borne by the front three saris-bearing ranks. The Persians - armed in traditional fashion - are simply fillers who will loose javellins and arrows and lend weight to the front sarisa bearers. When considered holistically, the evidence is consistent and surely points to front sarisa armed rows engaing in the pahalanx fighting.

You are also in a minority regarding the Phalanx having 'Half-file Leaders' - Connolly, Sekunda, Devine and many others all posit, like me, that Alexander's Phalanx had 'Half' and 'Quarter' file leaders.

No: simply that Arrian does not mention dimoiites or half file leaders. He mentions many other positions but not ever a half file leader to my recollection.

Anab, 7.22-23:

Then he proceeded to pass along this order himself and bade the others send it on--to ground their arms in battle line. [23] The men acted as their own marshals, and within a short time the hoplites had fallen into line eight deep and the peltasts had got into position on either wing.

The troops here are in a town (Byzantium) and are told to 'fall in' in their 'normal/proper' formations for a Parade and address by Xenophon in a paved square, and naturally they form up in files in 'normal/open' order, eight deep. They are not about to meet a foe, forming up in the last stages of an advance.Elsewhere, Xenophon tells us that for a fight they stood four deep i.e. 'half files' in close order. D

I will not go into that debate again. Simply - as Giannis has cleared up - "en machen" and "en taxeis" means to place in line or order. This is what Xenophon states on the two occasions. In your instance they are on parade and "eis machen" and "are not about to meet a foe", rather, go to dinner; at Byzantiun "en taxei".