Why should they have said so?
Malbim: They should have natural fear, based on intellect, due to Hashem Himself. If not, at least they should have feared punishment and withholding good. Also the heart and its physical powers should be aroused that via sin, he will lose physical success and find poverty and lack.
Why is it written v'Yoreh, and we pronounce it Yoreh?
Radak: According to the pronunciation, the verse mentions rain, and then details which rain. According to the Kesiv, He give rain at other times of the year, for gardens, drinking and other needs of man and animals, and also Yoreh and Moreh, which are needed for grain, each in its time.
What are "Yoreh u'Malkosh"?
Rashi citing Targum Yonasan: They are the first and late [rains. Yoreh is in Cheshvan and Malkosh is in Nisan - Ta'anis 5a.]
What is the meaning of "Shevu'os Chukos Katzir Yishmar Lanu"?
Rashi: They are seven weeks in which Hashem decreed two Chukim of harvesting - the Omer and Shtei ha'Lechem. Hashem arranges that the harvest is in its time (so we can offer them).
Radak: Rain does not fall at the time of harvesting, lest it ruin the harvest. This is one Chok of Katzir and its Mishpat (proper law). The other is that a good wind should blow at the time of harvesting, to winnow and clean [the grain]. It says "Shevu'os", and not Yemei Katzir, because we count seven weeks until Shavu'os. Therefore, the days of Katzir are attributed to Shavu'os.
Radak citing Targum Yonasan: In the time, He fulfills the oath; in the time of harvest, he guards for us.
Malbim: The seven weeks between Pesach and Shavu'os are the harvest time; Hashem will guard nature, that it will not rain then.
Of all that Hashem does for us, why does the verse mention the rains and Chukos Katzir?
Malbim: It gives one example each of two kinds of benefits. (a) Matters that we get directly from Hashem without any intermediary. It is as if he gives them to us each time through special Hashgachah. Rain in its time is not natural, rather, via Hashgachah. (b) Matters that we get via nature fixed from the six days of creation. It is normally hot and sunny in the days of harvest, and rain does not fall then. Also for this, we need that Hashem will not change nature due to our bad deeds, rather, He will leave nature like it is.
