The main purpose of conducting a reduced thrust (flex) take-off is to save engine life due to reduced low cycle fatigue damage, and therefore save maintenance costs. The lower temperatures, rotating and centrifugal forces will prevent the engine components to wear too fast and will prolong their life.  
Fuel savings are a disadvantage in most cases! In fact, you will have a slower acceleration so you will spend more time on the runway and at low altitude with a lot of drag (flaps) than if you were doing a maximum thrust take-off. The longer time spent at a lower altitude before reaching acceleration to your climb speed will certainly be less economical. However, the financial losses induced from the reduced-thrust take-off will be offset by the reduction in maintenance costs and engine spare parts.