From the last unit description of p. makedonikoi I read (EB1.1) I would say that they are not meant as a stronger alternative to normal peltastai... Rather something like "differently equipped hypaspistai" or those predestined to serve with the argyraspides... Young macedonian aristocrats trained at their weapons from youth and doing not many other things... So, they would take part in skirmish operations (moderately) and then, after retreating behind own lines and closing of the opposing battle lines, attack the enemy where the fight is becoming decisive.
However, in-game and apart from unit descriptions, they don't really have the stamina to serve as normal peltastai, do they? I mean, they once* actually slaughtered a somatophylakes unit of my KH army while the s. were in def-mode, fully formed up and well rested. Had to pull them out to avoid loosing the general and got a lot of other units decimated before they finally routed... So they should be a "ultra-heavy-nuclear-tank" unit, not some light sprinters. If they had a lot of stamina AND such a high def value, I would consider them game-spoilingly overpowered (see similarity to the *removed* Vasci shock troops, anyone?).

*City battle in Pella, the p.makedonikoi (2x) were the only units blocking that particular avenue and I needed six units to get past them... And no, I was not using any psiloi against them.