In reality, we are talking Gold Pass and Clan games.. Because these are the two usual occasions when excess books are received..
Both of these were implemented within the game as progression tools, therefore the likely reason for both doing clan games and purchasing GP is to progress ones account.
The only time one cant further advance their account is when they reach max hall.. Currently at TH13.
Anywhere below this state, and there is opportunity to use the items that are unlocked, thereby accelerating the progress of said account..
So SC are supplying opportunity for enhanced progression, not as a means for stockpiling gems or magic items..
Therein lies the disproportional differential between purchasing from the trader and redeeming gems on the same item as a trade off..
The former is the price for someone with necessity, and purchases the item as a singular transaction, and the latter for someone that obtains the same item as part of a package, and has no use for it.
Of course SC dont dictate said items must be used on receipt, nor do they insist on a given build style for ones account..
So folk can upgrade or not, at their discretion.. They can fully max or hyper rush each level at will..
But if someone expects to camp at a lower than max level, or otherwise upgrade in a way that prevents all progression tools from being beneficial at any given time, then it aint on SC in my view, if they find themselves in a position whereby they are having to trade magic items for a discounted return.
If you were max 13, I would agree with you, because they truly cant use those books.