Depends on how you plan to get emeralds from them. I'm not partial to smelting them (I reserve that fate for slimes). Farmer villager with a wheat trade + 10/10/10 wheat crop = PROFIT!!! And if you rotate the trades periodically (even if you have to spend a couple emeralds to do so), there's less chance of them 'burning out'.
I was imagining a couple of poor sods standing on a trap door above the smeltery, just waiting for your cackling laugh to end before a switch was thrown.