Actually, yes, I think it is sad if parents don't bother with school christmas fairs if (big disclaimer) there is no reason for them not to go. Fair enough if you or your dcs are busy - Tortoishell, with your sort of schedule, no way would I have made the christmas fair. Or if your dcs really don't want to go or you know through bitter experience that you don't get on with the parents and children at your school.
But otherwise yes IMO everyone is missing out - you, your children and the school - if you never got to school fairs and the like. Yes, they cost money, but a lot less if you just go for the last hour and take small change only.
How many times do you hear of parents wanting a school for their child that has a good sense of community? It is by joining in a little, going to school events etc that the sense of community is fostered IMO. It is not the only way, but is is an important way IMO.
I also don't agree that school fairs are purely for children's enjoyment. They are one of the times parents, especailly WOTH parents who do not do pick ups and drop offs, get to meet other parents socially. I see them as a natural opportunity to meet the parents of my son's friends, swap phone numbers, arrange playdates etc.