Andy, thanks so much for picking up the topic of color pickers (pun intended).
I'd like to discuss cases when accuracy is not as important at results. In this case there's a problem of abundance of choice. You don't have time to quickly and correctly pick one from the 16777216 colors. Plus alpha value? Come on! Whenever you're confronted with a photoshop/gimp-like color picker you either choose random color, or single color that's globally predefined in a palette. I also understand that very few 'casual' users maintain own palettes for consistency.