Consider this.
If we use the real-world dark ages as a point of comparison, those farmers are tithing to the local church. It is therefor in the best interests of that church to see the crop yields be as good as possible.
At a guess here, a village farmer with a wife or two, two or three sons, and a daughter or four, likely works, what ... six acres? Ten if he's moderately prosperous?
Using Conaill's numbers, that's 48 to 80 bushels. Let's call it a round 60, just for simplicity's sake, for the reasonably-average farmer's expected yield of wheat.
If he tithes, he gives 6 bushels to the church, having 54 left for taxes, rent, his own table, and so on.
If Plant Growth is cast that year, however ... his yield jumps by 1/3, to 80 bushels. He can double-tithe the church, giving them 16 bushels. After doing so, he still has 64 bushels left for himself -- 10 more than without plant growth.
IOW, he and the church are, in essence, splitting the extra yield the Plant Growth spell generates.
Sure, sure, 10 bushels doesn't sound like a lot. But ... that's just over an acre's worth of yield. If that church serves a (small) village with, oh ... thirty farming families ...
W/o plant growth, the tithe would come to about 180 bushels; just over 22 acres' worth, enough to easily feed 44 adults. WITH the spell, it would come to 480 bushels -- 60 acres worth, enough to feed 120 adults.
Now, at roughly 4gp/bushel (spoilage, middle men, whatever), that's an income of either 720gp/year, or, 1920gp per year.
The profit is 1200gp per year. I don't see churches who give a hoot not gladly tending to their flocks with this spell, actually!