My understanding is that the open hand bicep pose is a requirement in the Physique division, as it is supposed to be a more "feminine" way to pose. NABBA have adopted this policy between their (now defunct) Physique division ( IFBB/NCP equivalent to FBB) and Figure ( which IFBB/NCP adopted years later and call it their Physique division).
You will note that many FBB's downgrading to Physique will often "forget" and hit a clenched fist pose before reverting to the open fist variation which is a requirement in Physique. I wonder the judges ever penalise the athlete for this...
I have not observed the open fist pose used a lot in FBB contests, not more that it ever has, anyway, closed fist is still the preferred variation, and in still photo's is a major way of identifying if you are looking at an FBB or Physique competition.
I prefer closed fist, and wish Physique would allow the pose more often, but I have been surprised to see how the bicep shape can vary between athletes when switching between open and closed in the same "Hit". In many athletes, it is quite pleasing to see a transition between open to closed in the same pose, really shows off the development they have achieved, and gives a sort of transition of half power to full power with the move into clenched fist, a demonstration of muscle expansion. It allows you to take in the development and see different views and shapes = more interesting.
By the way, love the idea of Bikini, Figure and Fitness flexing Rob! But I think we will have to be happy with the back stage flexing by athletes from the non flexing side of the sport, as I feel the IFBB / NCP wish there to be an option for non flexing contests and bodybuilding meets.