What you're saying is the same thing as "If one outcome of this quest can have you end up with a set of Mithril Chain, but you don't NEED that quest outcome to obtain Mithril chain, then there's absolutely no point in the option of that quest providing you with Mithril Chain; it's completely insignificant. The only two options are that its insignificant, or that that quest is the only way to get Mithril Chain, in which case you're COMPELLED to make sure you get Mithril Chain out of that quest."
Trolls gonna troll. Either that, or it's just a poor analogy. Anyways:
If the (two?) paths of obtaining "Mithril Chain" are competing with each other depends very much on the requirements of both paths. Is obtaining the mail much easier pursuing said quest? Then that speaks in favor of completing the quest. Are the two about equal? Then both actually have a place in the game, and I never said anything else! It's a poor analogy because obviously, in this case, you did not implement a different mechanic into the game, not even a different item, therefore we can't say that any ressources went to waste.
Now if there was one set of Mithril Chain lying around in the wilderness, and one set obtainable through a quest, and you definitely will never need more than one set, then that might raise some questions as to the appropriateness of the mail as a quest reward.
Things aren't just absolutely necessary, or absolutely meaningless. That's now how it works.
An umbrella on a day it's not raining. Pretty pointless... under those given circumstances.
A merchant offering the best prices will always be the better choice than any other merchant.
Unless, like you suggested, we just add fluctuating prices, so you have to travel to every single merchant to check their prices. Except there's no fun to be had and no planning involved there.
Just like taking advantage of some price differences throughout the realm. If you do it, then you'll benefit, at some point. But you don't NEED the benefit. It just changes the rhythm of that playthrough.
I don't know why you blatantly keep repeating that a player with more gold will have no advantage over other players, only a "different rhythm" to his playthrough. I've already adressed this.
One person might play on Normal difficulty, and use 5 potions in one battle to stay alive (purely for example... I know this isn't necessarily how P:E combat will work). Another person might just make super-efficient use of their party (totally optional, as the first person obviously proved by getting through that battle with those potions) and only use 1 potion. Boom. They made better use of their resources than the other person.
I won't repeat myself 1000 times, but maybe 999:
Managing your party better resulting in an easier playthrough is broadly accepted. There's nothing wrong with it. Having an easier time because you faithfully trudge towards that out-of-the-way merchant who will pay a king's ransom for your rusty swords, not so much. It's just a test of patience.
BTW you economy geeks have ignored what I said about better prices only applying to purchases made by the player, not sales. How about that?
the point about resources might be due to the distance you have to travel to get the chain in the wilderness, think o it being were you would consider it being the merchant in question. does that change the relevancy of the quest reward? to some yes, others no.
point about gold making parties stronger, that wasn't disputed in the statement you quoted, only the need, which you have stated would make regional pricing lost on the player, which the quote was offering a reason as to why it would make a difference.
you are right, if there is no reason to go to a place other than some super merchant then it is just a test in patience and is a bad design. as far as backtracking, depends on the game, in BG there was plenty of revisiting an area due to some development or another, also the lack of need would just alter tempo as there wouldn't be the need to return as gold was plentiful enough to not need to backtrack.
as for the selling being all the same price, while buying being different, it was hinted at prior to the argument from me stating that merchants should never buy or sell at a loss. though it isn't the only method of ensuring that doesn't happen, but it is one method, and is perfectly reasonable.