"Doubting" the team doesn't make one a fair weather fan. Blind faith that the team will go undefeated every year, in every sport is not what makes a fan. That's what makes a moron. Hope for the best, sure. But its just fine to be realistic. These teams are comprised of young men who are imperfect and will lose. Sometimes more often than they win. 

A fan is someone who cares about the team. Watches the games. Attends some or many games. A fan wears the teams gear whether they win or lose. They can criticize the coaches. They can wish for more. They can be fans. 

I don't post that much on this board. But in general, if the majority of posters here were "fair weather", this board would not last very long. The fact that there are so many here posting, arguing, commenting etc shows the strength of BYU's fanbase.