I think that Mike Jenvey has given a very articulate answer to your question, and the Mad Scientist and mono have brought in other worthwhile considerations. It can be made to look like a very complex problem, but in reality you have the crux of the answers in the three posts mentioned. As an aside, we use a flex derate when possible. And that is often at quite heavy weights, it is NOT necessary to be lightweight. We have conditions, such as contaminated runways, de-icing fluid etc where flex derates are not permitted, but for the vast majority of departures we'll flex if possible. Originally, we worked out a pen and paper method, using pad weights, which got us a 'safe' derate. About 6 years ago we went to a computer programme which does the calculations to the nth degree. After one year of using the computer calcs, the maintenance department gave us a staggering reduction in engine overhaul costs. We do about 7000 departures a year on my fleet, and the savings resulting from the computer flex derates worked out to approximately $1400US per takeoff! And that was when compared to the paper calculated derates. With 7000 takeoffs per annum, that seems like a worthwhile savings to me. We have since gone to a 2nd iteration of the software which is yielding even greater savings. In todays competitive environment it can only be a dinosaur who would 'always use full throttle regardless'. Companies won't survive with that attitude. I am certainly not averse to building in a small fudge factor for me, the wife, the kids, and the whole song and dance, to ensure that I'm at a comfortable height when the end of the runway disappears under the nose. I don't see the need though, to be at 3,000 feet when I'm still over the concrete.
What must override the bean counters though, is that the whole flex temperature scenario must be prefixed with the thought that one accident will wipe out any cost savings for several years, therefore minimum aircraft performance must always be respected. And that is one of the reasons the four bar chappie gets the medium sized bucks (unless he gets the big ones with a national carrier).
To sum up then, use a flex derate system wherever prudent, while maintaining safety margins, and damn all bean counters.