It was put up mainly as an alternative from having “Suspense” as the default account, as many physical v’s accounting variations, e.g. resulting from a stocktake, can be without any specific cause, they just occur so the Inventory - Cost account becomes the “adjustment” location. Also, it eliminates the need to create specialised accounts for what could be inconsequential amounts/values after the netting of +'s and -'s.
This default however doesn’t preclude the user, as stated in (3), from being “able to choose the most logical expense account” as per your examples and others - promotions, donations.
But that is why it is the perfect location. “Inventory - cost” only receives allocations based on average cost not monetary valuations, which are an accumulation value based on purchase price history plus freight-in. Overtime the average cost value doesn’t have any direct relationship with a monetary transaction.
Therefore, when making Inventory adjustments (+/-) based on average cost it would be logical, in the absence of other specifics, to use both Inventory on hand and Inventory - cost accounts.
I would dispute though that a gain in inventory is an income. A farmer who has increased inventory due to natural activities doesn’t have an income from that increased inventory until it has been sold - yet most tax jurisdictions require that natural increases to be brought to account via a set nominal value which is taken up in the accounts via the COGS section.
Also, this propose modification upholds the Manager thesis of not causing the user to fight with the programme by forcing them to open specific accounts before they can make any accounting adjustments.