I may just be slow on the uptake, but I've never really considered the fact that eternal wands have a fixed cost for each spell level. Almost everything else makes you pay more for spells with costly material components or xp costs. The most practical application of that loophole I can think of is an eternal wand of identify. It costs 820 gp to buy it, but you save 100 gp each time you don't cast identify normally, so it pays for itself after 9 uses.