There are a few other factors that enter into this discussion specifically regarding the entire crew.
Firstly, the pilots and flight attendants have a limitation on crew day. This is partly by the company, but also by the regulatory bodies (FAA, Transport Canada). This limitation on crew day addresses the aspect of fatigue. Airlines do not have complete crews available on a moment's notice to take another crew's place.
Similarly, there is a requirement for a minimum amount of rest for the crew between duty days. This is a safety factor.
The second part is that the crew is probably not just flying on one leg. Often the crew will arrive at one location only to have a short turn around and return. They may also be used on a different flight when the arrive at their first destination. Thus, the whole network of scheduling the crew is affected.
The passengers only see the impact of the delay on themselves. They should be p****d off if the delay becomes excessive. We had a flight delay last year that could have been avoided had the aircraft been prepared a bit earlier. What we did not see is that the crew on this flight may have arrived late the evening before because the weather was bad, thus causing a requirement for them to report a bit later the next morning. Just think of the compounding factor if they have to sit on the ramp in the aircraft for an extended period of time.
It may also be that the weather at the departure airport seems to be OK for departure, but that the backlog of traffic caused by the weather will result in holding in the air either enroute or at destination. More fuel burned in the air = higher cost.
The airlines are in the business of providing transportation to the passengers. Those that keep this in the forefront can offer an explanation for weather delays, but they may not be able to do anything to change the situation. The good ones recognize this and take the steps to miminize the impact.
I don't think that a passenger bill of rights will do much other than cause ticket prices to rise. There will be a cost and we will have to accept that cost.