Sails are designed around masts with specific bend characteristics of which there are many as shown in

www.unifiber.net/masts-selector
www.peterman.dk/masts-all-imcs01.htm

I think I understand the concept of having a flex top - the stiffer more stable part of the mast is lower allowing good stability in the lower grunty part of the sail. The softer upper section allows the sail to exhaust well.

If that is the case why would you go to a hard top? What would be the benefit?

Maybe I'm confusing stiffness with curvature and the stiffness of the top section of a hard top and flex top is the same. Then I guess it becomes a sail shape design choice - you can carry more sail in the head, all other factors being equal, for a hard top.