A team's fans are more cognizant of that team's shortcomings than anybody else. If fans think an announcer is pulling his punches, they lose interest. They want a broadcaster to raise some hackles. But if you work for a team, calling every game for eight months a year, it's human nature to identify yourself with your club. In fact, I think it's honest when the announcer falls into the first-person plural.
Enthusiasm can coexist with honesty.
The best announcers love their team, even as they criticize their team. Of course, they also don't ever forget that it's sports. Bill Veeck was perhaps the one sports executive who was most in favor of a free and unfettered press. But one April day in Chicago, Veeck called in Jimmy Piersall, his announcer, who had made note of the rather obvious fact that it happened to be chilly at Comiskey. It's never cold at the ball park.