Marcus:
No, no-one would have questioned the decision to put Parmenion in charge - except perhaps Alexander, who probably would have been expecting such a command. But, as I say below, it's more a question of what the army would have thought of Alexander later on, had he not held a prominent position at Chaironea.
You're arguing from the point of view of someone who knows how things turned out. You know that the Macedonians won the battle, that Philip died only a couple of years later, that Alexander then became king, and that he conquered Persia. But Philip didn't know these things. What you're saying is: it was a fortunate decision for Alexander. What I'm saying is, this doesn’t mean it was also a wise one. Not, in my opinion, based on what Philip himself could have known in 338 B.C.
No, I don't think it 'weakened' the army at all. Even if the generals were 'supervising' Alexander, that wouldn't impair anyone's ability to control their part of the battle - after all, if Alexander's in charge, they aren't; and if they're in charge, then they're not supervising Alexander.
If you don't put the most capable person in every position, you weaken the army. Alexander was not, by a long shot, the most qualified person for that position. He was inexperienced and unpredictable, and could have made mistakes that the generals would have been unable to correct. And of course it burdened them. If Alexander was in charge (as I belive he was) and they weren't, they still had to supervise him. Had Parmenion been in command of the left wing, rather than Alexander, then this supervision would not have been necessary.
And Alexander was by no means inexperienced - he had campaigned successfully (at least once, that we know of) two years previously, on his own; and he had had a fair amount of training at his father's side. The amount of military training he would have received from the age of about 7 would have made it perfectly clear what he was and wasn't capable of, even without previous field experience.
He was ridiculously inexperienced compared to the generals that had to supervise him. One year's worth of real military experience is not suddenly a lot merely because the person we're talking about is Alexander. And regarding Alexander's military training, I'd say there's a rather great difference between fighting fake battles against other teenagers – if Alexander did even that – and commanding the left wing in a battle that Philip had reason to believe would be his most difficult one so far.
But, ultimately, it still goes back to the "way things were done" - no, Philip didn't have to put Alexander in charge of the left wing; but had he not done so, then Alexander would not have "gained his spurs". If the sources and most commentaries are to be believed, the army had such a strong say in who the next king would be, that had Alexander not been given the chance to prove himself to the army, they might well not have ratified his accession. Therefore I cannot see how Philip could have avoided giving his nominated heir a chance to prove himself.
I'm not arguing that Philip should have avoided it. I just think that he should have let Alexander gain more general military experience before he allowed him to command. He could have just let Alexander fight in the battle as a regular soldier, without command responsibility. Alexander would have had to "gain his spurs" as commander eventually. But he did
not have to do it at the age of 18, in a battle that would determine the future of the Macedonian empire. By allowing Alexander to do so, Philip took a great and in my opinion unnecessary risk.
And again: I'm arguing from the point of view of what Philip could have known in 338 B.C.
Had Alexander "missed" the battle, what would all those beefy, macho and ever-so masculine phalangites have thought of him? I can think of a few choice words ... but once he was king they wouldn't have followed him to the toilet, let alone across the Hellespont and as far as India!
This is irrespective of whether Alexander actually commanded the left wing, or was nominally in charge of it.
But...what if the battle had been lost? And what if Alexander had caused the defeat? What would the surviving soldiers have thought of him then? And would it still have been a wise decision to let Alexander command?