I can start.
Currently for oilseed and grain drying and for the heating and cooling of livestock barns, there just isn't another technology commercially available at any sort of scalable, economically viable level.
As for a rebate versus an exemption, farmers during the season need to be able to invest in their operations. When they actually have more money on the farm, they make more investments.
These days, the average combine to harvest your crop is now over $750,000. Those were major capital investments if you made them, but there's no alternative to natural gas and propane at a scalable, economically viable level.