Throughout the series, it becomes known that Captain Sisko has a love for the game of baseball. What better way to "make real" his affection for the game, then to actually have one played on DS9.
Those who thought the episode was silly either was not a fan of baseball, or is simply a negative person who can't see the lighter side of things.
Not only was the idea of the episode awesome (I would have rated it an 11 if tv.com would allow it) it was brilliantly written.
A silly episode would have been written in a predictable way. The way events unfolded during the game were surprising and unexpected. The victors of the game and the game's stars, mirror the series' twists, turns, and unorthodox style dissimilar to the other Trek series. The way it ended was a gem!