Guess that there exists a variety of considerations here ..
(a) the basic requirement is that the takeoff meets the Type Certification and, by inference, the relevant Design Standard
(b) rated thrust is fine, and available for those who want to use it
(c) derate/flex is fine, and available for those who want to use it when it is available from a certification (AFM) point of view
(d) no matter how anyone might want to skew the story, jets don't like the high end of the temperature range .. a few degrees cooler reduces maintenance/operating costs. I am sure that Mutt can comment on this ad infinitum
(e) use of derate/flex is only available when rated takeoff is not limiting ... otherwise rated is mandatory
(f) use of derate/flex doesn't have to be taken to the point of making a non-limiting takeoff limiting (which appears to be what is upsetting some of the posters). Although I don't know what they do now, in the early days of flexing, Qantas imposed an arbitrary 1000 ft pad to the figures to keep the crews happy (at least that was Wal Stack's story at the time ..) Similarly, any operator can impose whatever fat it may choose corporately. I did the sums for one operator for many years and the Chief Pilot and I, in co-operation, applied a variety of considered pads to various runways with corporate blessing .. the corporate side was quite comfortable with the idea of balancing the mighty dollar against maintaining a good argument for any legal conflict.
(g) equally, an operator may run the derate/flex to the nth degree to squeeze the last bit of advantage out of it .. so long as the TC/design requirements and any operational restrictions are met. Many of the posters whom we read in PPRuNe work for operators who do just this.
(h) would I have a problem paxing on an operator which scheduled limiting derate/flex ? .. Certainly not just for that reason .. provided the overall operational risk philosophy was reasonable ...
(i) certification doesn't address a bunch of multiple failures .. in the event of such a circumstance, the crew is called upon to exercise its great knowledge, experience and skill at a moment's notice (not too sure where this leaves a minimally experienced crew .. but that is the way it's going these days) .. and then the Captain has to try and justify these decisions at the eventual enquiry in painfully slow and intricate detail ... For those who have had no experience of such inquisitions, the recent Australian military BoI into the Nias mishap makes good bedtime reading. Another which comes to mind was the KingAir 200 (?) crash at Sydney years ago ... an Ansett captain operating into Sydney at the time (and patently unrelated to the crash) sustained a very lengthy cross examination at the enquiry. I am sure that most of us can cite numerous such cases.
I suspect that ssg may not have had any/much experience with derate/flex takeoffs.
For those who have, and seek to champion its benefits .. please keep in mind that folk like ssg's operator are perfectly OK to operate at rated thrust if they wish. Many years ago I worked for a government operation which, amongst other Types, operated a Dart powered bird .. which they operated wet takeoff routinely .. often running up to takeoff power on the brakes .. even when empty from long runways. Didn't do the maintenance/operating costs much good but that was their choice .. our taxes at work. Centaurus could add much comment to this as he was a Captain for that operator at the time and did his best to inject a bit of commercial commonsense into the operational philosophy .. with not a great deal of success.