You keep saying we need "spies". Show any indication on this forum of people even talking about using "spy alts". Not speculation of them, actually saying "I'mma use a spy alt."
Bank alts actually date back to Vanilla because, guess what, THERE IS NO GUILD BANK. That wasn't added until TBC.
"Blizzard-supported guild banks were introduced with patch 2.3 to help reduce player-created guild banks, which were prone to Terms of Use violations due to account sharing."
And if I remember right, the max bag size in vanilla is around 20ish with 7 extra bank bag slots purchasable. That's 140 MAX extra bank size plus the built in 28, rounding out to 168 bank size. Now many professions, be they crafting or gathering, use a lot of components which IIRC go up to stacks of 10 in Vanilla (food goes to 20). That's 168 unique stacks of mats of 10, which you usually have MANY stacks of (most I had was 13 stacks of Strange Dust). Most recipes for high end gear/enchants/potions use about half a stack of each mat at the least.
BUT WAIT. We forgot the most important thing, gear itself. See in Vanilla (and BC..and LK to some extent) you need the best gear of specific resistances to do specific raids OR EVEN parts of raids (Naxx.. dear god, Naxx). That's 8 pieces of gear for each resistance (more if you have weapons with resistance), and there's 4 main ones: Shadow, Frost, Nature, Fire. 32 slots needed to hold your resist gear when you're not using it. That leaves at the most 136 slots for your crafting mats and assorted items like potions, RP gear, toys, etc..
Starting to get it through your thick skull why a bank alt would be useful? It can hold all the unbound stuff and potentially all the crafting mats to give you some form of relief.