It's really important to me because a great soundtrack can push a game over the top.
What would Halo be without that wonderful score to add atmosphere to important parts of the game?
What would Zelda be without the main theme (though Big N doesn't use it much anymore, the bastards)?
I don't even have to mention the Mario theme.
BioShock, Silent Hill, and many of the most successful "adult-oriented" games use music to engage the player and heighten the mood. Without that music, it's a lot harder to achieve an epic feeling in-game (probably stemming from our love of film and John Williams over the past 30 years). Gamers have come to expect the score to match the experience, foreshadow events, etc.
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