There is one other point that has not been spelt out. All the disruption to sleep patterns is one thing; all the disruption to one's private life due to roster changes is another. The tiredness that the duty times cause is excentuated because there is so little quality time off with friends, family hobbies etc.
This erodes morale, as has been stated. However, when you have crews going to work in a pi@*ed off mood, and the source of that attitude is the employer who is identified by the uniform and the paint job on the a/c, I believe there is a safety aspect to that.
Much of CRM courses I've been on tell us to be careful about our inner attitude and any destabilising effects that close problems may have on our decision making when flying. Becareful, and if in doubt, perhaps take yourself off a flight. If the source of the anger is the very work enviroment itself (last minute roster change etc. etc), surely that is quite significant in the safety world??
In my last ops manuals there was the clause about management being aware how their actions might effect morale and the effect that would have on Flight Safety. Fine sentiments, but hollow words. If they paid any head to them there would be much better rostering policies. I'd take a stab at betting that the most common gripe amongst crews is not money, or duty hours; it's the roster patterns and the constant changes that really erk them.
Low morale is countered by professionalism. There has to be a breaking point when it doesn't work out. Add tiredness and low experience into the equation.........................