There have been plenty of discussion threads about this very topic in Tech Log so a quick search there should bring results.
Not sure of an unlimited runway length, but I certainly know of a situation where the runway is short and higher MTOW can be achieved with a derate. In my experience a short bodied TriStar(-500) which has a very high Vmcg because of the reduced rudder arm, could gain a 20T weight advantage out of a (relatively) short field by derating thrust. This permitted a much lower V1 and therefore allowed the a/c to reach it quicker (even allowing for higher weight and reduced thrust) and therefore was able to stop within the runway length available, rather than rushing down the runway to achieve a V1 (higher than Vmcg) and roar off the runway with no useful load whatsoever!