So the anti-Flex/derate brigade's argument is, always full-rated unless an error has been introduced due to manual errors in calculating.
So by that rationale what about RVSM ? Maybe we should have min vertical separation of 2000 ft, maybe more, just in case someone mishears a clearance/put the altitude into the MCP incorrectly and it isn't cross-checked. Actually best make that 4000ft just to be safe.
What about departures at MAUW or MZFW. Surely they have to carry a greater level of "risk" as they are at the very edge of performance. Let's offload 2 tonnes of freight, actually make that 5 tonnes, just to be sure.
The fact are that there is no greater risk of Flex/Assumed Temp departures when done correctly. All figures are calculated that even the worst case scenario (engine failure at V1) is catered for as far as runway length goes, with the necessary safety margins.
If your argument is to use full-rated thrust to remove a possible error being introduced (incorrect V speeds or thrust set) then think about it this way, maybe gear shouldn't be retracted after takeoff, this will reduce the risk of the crew forgetting to lower it before landing.
How does that meerkat put it, oh yes, "Simples"