Pilots and Flight Attendants get blamed for a lot of things out of their control because we are the most visible link to the company when passengers board the aircraft. We share the frustration because most of us want to do a good job and get you to your destination on time.
I have had my plane taken many times to cover another flight and waited also seeing our intended aircraft taxiing out and we have a two hour delay for the rescheduled aircraft. As the pilot I complained to the Chief Pilot that this happened often enough for our lower priority destination which I normally flew that I felt like we were flying a standby aircraft we could use if no one else needed it.
He appologized but said dispatch has to reschedule aircraft to keep things flowing as smoothly as possible.
Always leave time when flying for these kind of delays.