I agree with Field In Sight.
Over time the fuel saving is considerable, let alone the engine wear. Also, if you lease the engines you recover a premium from the company (who own the engines) for taking off in Flex and you may even incur a penalty for not using Flex in ideal Flex conditions.
The aircraft takes off very well in Flex and TOGA should be used in conditions only where increased performance is necessary.
Thats why the FLEX detent is there
