Excellent post pannin4gold. I feel very strongly about your assessment that being a fan involves inviting irrationality into your life--thankfully, we're not Cubs fans or else this would be a
really miserable place to be.
My take on it is that we are fans because we want to be. Whether or not our families, friends, or ourselves as individuals took a liking to this team is irrelevant because we find ourselves in a position to share in the collective fortune/misfortune of vicariously winning through the 49ers. I put on my my Gore jersey (almost) every Sunday and attempt to watch the game, if I can, because I desperately want to see the team win and have a shot at being the best at what they do. That's the exhilaration of being a fan: it's an organizational way of being part of something that we have a developed interest in.
Now, while I approach the possibility of a playoff run with a bit of trepidation (The margin of error is slim, it's hard to be skeptical), at the same time, the hope of the unexpected miracle (again, irrationality, but I'm a fan) drives me on to wish the team success. The 49ers aren't doing their job if they tank the season: their goal is to win as many games as possible and to be the best team in the NFL. Whether or not it's under Singletary or some other coach and with Alex Smith or Troy Smith or whoever is at quarterback is irrelevant, because I want them to succeed, plain and simple.
Maybe we get into the playoffs. Maybe we don't, and Singletary is shown the door. But so long as the team is his to coach, I will hold out hope every game day that he comes away with a win. I'll be happy for him and the 49ers.