The Makedonian phalanx -- why such depth?

[MeinPanzer]

That's a tricky question, Ruben, and might lead us into yet another debate !! (
I'll try to be brief here.
Suffice to say that what the manuals are trying to do is divide Infantry into three basic types: viz, 'Hoplites'/Heavy infantry/Infantry of the line, who are shielded, armoured and either spear or pike armed ( Macedonian style) and fight hand-to-hand; Light Infantry are 'the opposite' to heavy, being un-armoured,shieldless and who use slings, arrows or javelins to fight at a distance. In between are 'Medium troops/peltasts' who are more lightly equipped than the Heavy Infantry, and who can both fight at a distance/skirmish and hand-to-hand if necessary. As described elsewhere, the thureos was adopted following the Gallic invasion of Greece. The thureophoroi who in effect succeeded the peltasts in the third century are sometimes termed euzonoi ( light troops) and fulfill the role formerly taken by peltasts.
For example Plutarch, describing the Achaean citizen troops following the adoption of the Thureos says they skirmished from a distance, but were ineffective at close quarters, and that their tactics were peltastikes (peltast-style). They were evidently not a great success as 'hand-to-hand' fighters, and it is probably significant that those Greek poleis ( cities) that experimentally replaced their Hoplites with Thureophoroi usually quickly re-equipped as a 'Macedonian-style' phalanx. By the 2nd century BC, the typical mercenary seems to have been mainly this type.

I should first note that I am only working from Asclepiodotus, as I think that his manual is the only one that can be verifiably linked to any true Hellenistic military practice.

Firstly, Asclepiodotus does not say that the psiloi are shieldless. He says merely that they A) fought at a distance using "javelins and slings, and in general those missiles which we call 'long-distance missiles'" and B) did not wear greaves or cuirasses.

Secondly, Asclepiodotus is explicit about the peltasts actually carrying peltai, as he states that they are lighter than the hoplites in part because "the pelte is a kind of small, light shield." These are clearly the more flexible, often elite troops that we find in Hellenistic armies and who did often form up with the phalanx, but who could also fight in a looser manner.

As you yourself say, we have some mentions of thureophoroi from the literary record as euzonoi. Since Asclepiodotus' category of psiloi doesn't exclude thureophoroi (since the commonest equipment we see them with is thureos, helmet, sword, and spear or javelins), but his category of peltasts explicitly does, it is only logical that the thureophoroi are included in the former category, and not the latter.