Probably stating the obvious here, but since the WE cost is so low for the first 3 stages of the rare tier, it’s actually the most WE efficient way to fill your regular monster chest.
Each of the first 3 stages you have a guaranteed 9 monsters for 1 WE, so you only need to expend 12 WE to fill the chest. Farming 8-7 instead, you expect to see a minimum of 11 and up to 14 monsters, although in my experience the average hovers around 12-13. On a lucky day I can get it done with 8 runs @ 24 WE, but if RNG veers slightly off course I need 9 runs @ 27 WE.
Sure for those 12 WE you used to farm the event level you’re missing out on potentially 6800 food, 13k iron, a bit of recruits and XP, and slightly less loot than if you were farming 8-7. But IMHO the trade-off is very much worth it for saving 12 WE. FWIW I’ve gotten troop/hero drops and backpacks from the first 3 stages of the event levels, so it’s really not that bad.