Patents serve the same purpose in farming as they do elsewhere: it makes it worthwhile for companies to invest the hundreds of millions of dollars it costs to drive crop yields ever higher.
I share your moral outrage at the idea of patenting life, and all my own research as well as all the genetic technology produced by our consortium on the C4 rice project will be made free (as in beer and speech).
However, agriculture relies on agricultural companies, and they need some way to make money. If people can just take their technology and grow it again themselves, there is no way for them to recoup their investment and we simply won't have better crops.