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Last I heard, Chuck Armstrong makes these calls in-game, and without the slightest rancor I would assert that he's prouder of the "ballpark experience" in Seattle than he is of wins on the field.
This factor has been part of the fabric of baseball in Seattle. Like Woody said, they want (the ballpark) to look good first, and win second. Oh for a Billy Beane who would play in Cheney Stadium if it meant a World Series.
You could make an argument to close the roof in April and May, any time it's at all cold (40-49 degrees) or threat of drizzle, but it ain't gonna be the Mariners listenin' to it.