In my opinion, the low cost airlines in particular have provided a means of countering the effects of 'fatigue' by recruiting and then training high calibre personnel to a high standard.
Nevertheless I once 'jumped ship' when extremely fatigued (with the co-operation of the Captain who was a CRM trainer and appreciated the issues) by stating I was suffering from a 'tummy upset'. Heaven forbid the word 'fatigue' was mentioned.
The airline's view being that if the company operates to CAP371 then it must be the pilot who is to blame for not getting sufficient rest (and presumably for flying when fatigued if the unthinkable were to happen).