According to Boeing we save around 40kg of fuel each sector where we can do an unrestricted climb at full climb power instead of using the automatically selected reduced climb thrust.
But that assumes an UNRESTRICTED climb.
When did you last have one of those in e.g. London TMA?
Hence you reduce climb performance as well, to reduce rate of climb without the crew intervention by requirement of using V/S continued througout the climb.
Reducing the amount of power up/down required with each clearance.
Powering up further requires more fuel than 40kg per flight saved with an unrestricted climb.
When unrestricted climb is possible... use full climb thrust!
Last argument in favour of reduced thrust use on takeoff, it LIMITS the damage when a failure occurs (such as effects of a bird strike when compared at a lower or higher speed engine core and fan)