Me too! When I was a better younger surfer sometimes I'd get really pissed at myself blowing rides all day and eventually if not too many people around I would punch the nose while sitting on the board. I hoped to knock the nose clean off, but it would just sit there vibrating at my uncoolness. Doing so I noticed related resonance and tried to judge a few boards by their vibration factor.They have a different resonant frequency - that's how I measured the flex.
I figured if a board had a duller this, it had too much flex and or less potential tenacity if you will, however over the years, definitely had good boards with a full thud reaction.
I've always felt flex in positive ways, when applicable.
I do think the average surfer, and even board conesiuer afficiendo types are tripping little more than they realize over especially minute fin details. Not saying it's no big deal, or that people can't discern different fins, just that I think there are many listening and paying attention to fins, board builds, leash strings etc, that I've seen surf and they have no clue what a good archer do yet they go all in on the arrow feathers contemplation.
It's a good thing anyway.