I'm not sure why there is such a huge concern with calling in fatigued. One was rested and prepared to fly the day as it was originally scheduled, but unforeseen circumstances forced delays that fatigued them to the point they are not safe to fly. I don't know what the companies failure to pay that situation record is, but I'd guess it's fairly low.
Don't fear the fatigue report, just fill it out and if delays from IROPs/weather/broken airplane caused you to be tired, don't fly.