My understanding is, as other have stated, that they are using the gemhearts to encourage the growth of their crops. I hadn't considered a fabrial being involved, I assumed it was just a way of using stormlight that the listeners knew of but humans have forgotten.
With respect to little gems also working for this, yeah they would, but the alethi could also use little gems for everything they use gemhearts for, gemhearts are just better because they hold more stormlight. Also I don't think the listeners have any way of aquiring new gems other than gem hearts, there aren't mines on the shattered plains.