I agree that it is too static and that two dimensions are simply not enough to offer an even basic layout of a character's morale. However, what most of us are discussing are actually lamentations on a very high level. We, who understand that it is perfectly possible for two paladins to get in a fight, that it is perfectly possible to have a good character kill a non-evil (or even good) character because he has someone to avenge, that it is even perfectly possible for an evil character to have close friends and loved ones, we would most certainly never need a construct called "alignment".
But there are other groups out there. Not only my beforementioned new players. There are groups who have "that player" who takes murder-hoboing too far despite playing a paladin. Who betrays his own party. Or who is labeled "good" but does so many "evil" deeds that he wouldn't qualify for that alignment any more. There are groups where a DM wants a certain tone ("evil campaign" or "heroes" or "thieve's guild") and ruling out certain alignments is the easiest way of making sure every player in his group understands what the campaign is about and what their characters should be like to fit in. Heck, even my group which has evolved a lot over the last few years used to have a (very nice!) player who occasionally wanted to switch characters and regularly wanted to inject evil characters into an otherwise heroic group.
Having a defined "value" (and I even toyed with alignment points at some time) that can be invoked and defined is a tool for a GM or a player to describe what is and what is not appropriate behaviour. Not that alignment should be static. I fondly remember a (Planescape) mercykiller PC who changed both alignment and faith over the events he had to witness during the Time of Troubles, who came to understand that justice can never be just without a compassion for others. At the end of the campaign, said PC became a famous, altruistic peacemaker and diplomat.