Bader made a distinction between tiredness and fatigue, the later resulting in significantly impaired reactions and performance by an individual in his duties. If a normally fit and well crew member is within the FTL and having taken due care to utilize rest periods as intened, reaches a point of 'fatigue', then it is possibly a failure of the company's FTL scheme. That should, in every case, result in an MOR/ASR, and a subsequent investigation as to why.
Although, I can understand why some would be reluctant to file an MOR, failing to stand up and do so may be masking a bigger problem. Fatigue should be reported as an occurrence and any company worth it's AOC would honestly look at the issue, not least of which it is on record with the CAA and the Company's flight safety people.
Further, crewing and ops officers should be trained in Human Factors/CRM and Basic Aviation Physiology and Health Maintenance and the use of phrases like, ' are you refusing this duty' when related to tiredness or fatigue should never be used.