Assume you are advised by ATC or a preceding aircraft of reports of strong windshear after take off. On that basis you elect not to make a reduced thrust take off as a precaution. If de-rated thrust was originally planned (as well as ATM) would it be considered good airmanship to therefore change from de-rated thrust to full rated thrust in case you need it to counter the expected windshear?
After all, you don't know exactly how dangerous that particular windshear may be and you would look a bit silly in a court of law if the prosecutor put it to you as the captain, that if it was good enough for you to cancel the assumed temperature power because of expected windshear then surely it would have been prudent to delete the de-rate setting as well? After all, it is only money we are talking about in terms of the advantages of various reduced thrust ratings - and not primarily flight safety...