I think people are too anal about having to be the best spec for every fight. They are not far enough apart to make a difference, and switching around constantly probably causes more mistakes to be made, having a negative impact on performance. WOL parses are also likely to make the picture worse because parses are more likely to be made by the most high end players, who are most likely to use the best theoretical spec, skewing the numbers toward that spec naturally.
I mean, if you like flip flopping specs and gear and whatever all over the place, I support that totally, but if you are only doing it because ZOMG the interwebz said Aff/Demo/Destro is better, its probably not worth it.