Many successful websites (Google, craigslist, ebay, wikipedia, myspace etc) are both highly functional and divergent from "good design practices".
What I find troubling is that I don't believe any of those sites would have been constructed in their successful form by a design professional. We don't fully understand how people's perceptions are shaped by the presentation of information.
Having appropriate whitespace/fonts etc produces a clean looking site which conforms to a very particular aesthetic which has no proven connection to the success of a site and may substantially harm it.