RE the sugar: it isn't so much that filtering it through sugar is necessarily bad. It's just that, with good absinthe, you well may feel like it isn't adding anything (although you also may feel that it is). In other words, with good absinthe, it's simply a matter of preference. But not necessity. (Just like some people like sugar in coffee or tea, and others don't.)
As for the flaming sugar ritual, as far as I can tell, that's a recent invention by goth retards, and adds nothing but idiocy.