Testing Designs (and Designers Testing)

Jakob Nielson’s latest Alertbox is the longest-read enewsletter in my list. In the latest Alertbox, he points out “designers can get so caught up in their own theories about how users ought to behave that they forget to test for cases in which people behave differently” Could it be that the mechanics of setting up multivariate testing neutralizes this “worry”? Should variable definition be more of a part of the web design process?