Sarek does not defend his wife from the comments made by the Council because he is a junior member of the body and probably would have been scolded for making such an emotional outburst in front of his colleauges. The audio book and book kind of hint at this as well as flesh out Sarek's character. Indeed, it would have been illogical for Sarek to have defended his wife even at the Councillers racist remarks. I find it more surprising that Sarek does outburst when Spock says he declines the Council's offer of admission to the Science Academy. At least he does in the audio book( Zack Quinto does an awesome job reading it!).