They're important to my purchase decisions in that I'd prefer my games to not look shit, and beyond that, it is of course very desirable to get a certain wow factor from a game's visuals. I mean why wouldn't you want that?
If I were to make a list, they'd be somewhere pretty low, but that doesn't make them not important. Great visuals can do wonders for immersion, and the like.....really helping a game come alive.