Lots of theoretical handwaving. Not a lot of data.

I can tell you that the stiffest frames I have are the ones the feel best in a sprint or other out-of-saddle effort, while the least stiff don't encourage that kind of effort. However, I'm also always aware of which bike I'm riding, so I can't dismiss expectation as a variable. And in the end, I don't need to because it doesn't really matter whether or not stiffness of the frame 'doesn't really matter' because somebody's got a hypothesis (it's a hypothesis, guys, not a theory).

Hell, I've even been told that stiffness of the frame doesn't matter for comfort and compliance because of some guy's hypothesis, but I still find myself seeking the least bumpy line with some bikes more than others, and some bikes on the same roads kick me up out of the saddle more than others. So I'll stick with my observations, because observation is how you determine whether hypotheses are correct anyway.