So the goal is to keep handy spells like Detect Evil working, right?
Let's look at the real world.
We know some people are evil. Watch the history channel when they show stuff about criminals and Hitler's friends.
So aside from people with odd views of alignment, we know that Evil happens and that there are Evil People.
A general rule of opposites apply and we usually assume there are Good people. Nobel Peace Prize winners come to mind. Activists who go to crappy countries and get shot while handing out food.
We also know that there are people who don't fall into those extremes, who we still often call evil or good (most people call themselves good, though they're not as dedicated) and most people would call the guy who raped their friend (1 in 5 women are raped) evil.
So there's varying degrees of good and evil. If you were so inclined you could make alignments for:
Saintly Good
Mostly Good
Kinda Good
Huh?
Kinda Evil
Mostly Evil
Fiendishly Evil
And then there's the Law/Chaos thing. I know for a fact, some people are very lawful. These are the people who argue with game rules and absolutely cannot deviate from the rules. And if its not written, they can't fathom that anybody could just make a decision. And then there's the people who just don't make any sense at all. I'm assuming they're the chaotic ones.
And unfortunately, we all fall into aspects of good, evil, lawful, and chaos everyday. We may tend toward one set of behaviors, but we are seldom one alignment all the time. The honest man who goes to church and helps out at the food shelter, but uses the company printer to print his D&D characters is STEALING!
So how do you model all of that, and all of that flux? Or do you say screw it, There are 9 alignments and they reflect the way you act most of the time, or are a summation of all your actions.
The more important thing is, alignment isn't an average. If I act nice all through 5th level, then I start maiming and pillaging the day after I level up, then I'm probably now evil. its more of a pulse rate on how the character generally acts.
Janx