The Gemara talks about how we know that Yoreh is good. Why didn't the Gemora prove it from the context of the actual Possuk in Shema. "If you will keep all the Mitzvos and love Hash-m etc, Hash-m will break rain in its time Yoreh UMalkosh and you will gather the produce etc you will eat and be satisfied". Obviously amongst the list of all the Brochos, Yorah must also be good?
Many commentaries deal with this question, the earliest being Rashi (DH "Hachi Garsinan") who explains that we would think that the Pasuk is stating that "v'Hayah Im Shamo'a" you will only have good. If not, v'Nasati..Yoreh u'Malkosh." The Maharsha already states that this is difficult to read into the Pesukim (which seems to continue with many blessings), and therefore points out that the Pasuk states "b'Ito Yoreh u'Malkosh" -- "in its proper time Yoreh and Malkosh." This possibly implies that Yoreh and Malkosh, unless perfectly timed, are in general a curse not a blessing. [For another explanation, see the Yavetz.]
Wishing you only blessings,