The very definition of "fair weather fans" is that they show up and want to be a part of a winning team. When the team is losing they are nowhere to be found. I can't say that Bibby is wrong.
I'm just saying that I think if a fan doesn't go to games when his team is struggling to win 30 games, that doesn't necessarily make him a fair weather fan. Maybe he's protesting his team's inept decision making by not supporting them monetarily. You have to remember that Danny Ainge was widely considered to be running that team into the ground up until this summer.
There are a few Kings fans who are members of this board, but have pledged not to go to another game until the front office commits to being competitive. I wouldn't call them fair weather fans.
Now, the bandwagon always grows dramatically when you have a turnaround like the Celtics had this season. I saw it here in LA when the Lakers started winning titles. There are a good number of bandwagon / fair weather fans in the mix there, but I don't think it's completely fair to criticize those fans who have been following and supporting a team for decades (which pretty accurately describes a good portion of Celtics and Laker fans) because they refused to spend money on a poorly designed product, and now have gotten behind their team as the decision makers begin to put together a much improved product.
It's like if you always go see a particular actors movies because he always puts out a good movie. Then he starts putting out crap for a couple of years, and you decide not to continue supporting him until he starts putting out better movies. When his movies improve and you start going to see them again, does that make you a bandwagon or fair weather fan? I'd say no.