In this recent New York Times article, Robert Lane Greene, author of You are What you Speak, takes the descriptivist side, and Bryan A. Garner, author of Garner’s Modern American Usage, takes the prescriptivist side. In the course of the long exchange, they discuss the which/that “rule” for restrictive relative clauses and the question of whether native speaker’s make mistakes:
http://www.nytimes.com/roomfordebate/2012/09/27/which-language-and-grammar-rules-to-flout