I myself have zero tolerance for fair-weather cast and crew. Over the years I've lost count of how many times I received a desperate call from someone in a production asking me to either do an emergency rush job on a prop or F/X gag or be a last minute fill-in for a crew position because someone bailed out of the production. More often or not, I come to learn that the person I replaced had a history of quitting from the get-go.
I understand that a lot of producer/directors tend to have their buddies on crew with them. I'm no exception here, as some of the members of my crew have worked with me for many years and are counted among my closest friends. Unfortunately, there are many people who expect that there is more of an "understanding" and higher tolerance level allowed when it comes to people who are friends with the Producer or Director.
Regardless of how close we may be, everyone who knows me well has no doubt that if they quit on a project that I'm supervising that department on, they'll not find me waiting with open arms if they reconsider and wish to rejoin that crew. Knowing this, people close to me will talk to me about assigning them to a minor position rather than quitting altogether and all is well.
When I decided to move forward with Predatory Moon, my husband was very honest with me about the time he would be able to commit to the project. While I've been his right hand on almost all of the productions that he directed, he simply could not return that favor and I certainly didn't hold that against him. Given the nature of the film, I asked him if he would be willing to handle the stunt scenes and assist me in the workshop with some building projects on the weekends, and he agreed to that as it worked with his schedule.
I'm not completely unsympathetic to people who need to take a leave due to personal reasons or conflict with work. If people talk to me about their needs, I certainly do what I can to accommodate them. I've had assistants who have had to duck out on me during pre-production to handle personal problems, and I've allowed them to come back to help out during production if they were available to do so. But this happened only in cases where the assistant came to me with the hardship and asked if such an arrangement was possible.
My no-rejoin policy for people who just up and quit on me is rooted in very simple logic. If someone doesn't have the respect or consideration for me to talk to me about options before leaving, it tells me a lot about what I can expect from them in the future. I deal with enough stress on a job without having to constantly worry if the summertime soldiers on the crew are going to show up or not on any given day.
I've asked other producer/directors why they would allow someone (especially in a key position) back on their crews who demonstrated such lack of respect or responsibility. The responses had to do with either people who quit on impulse during an argument or those who the director felt were "irreplaceable".
I've dealt with impulse-quitters before, and I don't hold them in very high regard because I feel the act is intended as a kick in the teeth. Personally, I don't keep the company of folks who routinely kick me in the teeth and I do everything I can to discourage that sort of behavior... including refuse to allow such a person to re-join my team.
As far as someone being "irreplaceable", unless that person is the one in control of the funding or a legal partner in the production company, I have yet to see a production where anyone (including the director) could not be replaced. True, it might push the production back for a little, but ultimately the productions were much better off in spite of the inconvenience.
A director I've worked with lost both his DP and his lead actor toward the end of the shooting his project, and he proved the point about how no one should be considered irreplaceable by bringing on a new DP and a new lead and completely re-shooting his film. The rest of his cast and crew (myself included) were happy to come back and do it all again minus the two deserters.
Perhaps some may think I take too hard of a stand on this, especially when it comes to volunteer projects. However, I sincerely believe that it is important to be able to trust your crew to see the job to completion and be able to make the hard decision to lock the door when someone gives you good reason to suspect that they can't be trusted to do so.