"It's the default setup in Rokugan, for instance -- you have Evil clans like the Scorpion who are ultimately on the same side as the Good clans, because they're all part of the same empire and fighting the Shadowlands." -Hong
And in Rokugan, they mention getting rid of alignment altogether, which is a neat thing.
Although, in Rokugan, I would argue that Honor/Dishonor is an absolute. It is dictated by the Celestial Order.

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I guess it all depends on how you see the "DnD reality" for your campaign.
I think the reason why there is a tendency to have Good/Evil be absolute is because that makes for an efficient game where everyone knows where they stand.
The problem with the relativistic morality can be summed up by this example:
An Evil Demon casts Detect Good on another Demon. What happens?
Does the Demon register Good or Evil?
If it registers Good, then the "Good" Demon has traits in common with the other "Good" Demon and they have certain traits with everyone else in the Lower Planes.
To that Demon, everything else is "Evil".
And if you want, it doesn't even have to be two sides. It could be an infinite number of groups of people who all think they are "Good" and everyone else is "Evil".
Which is where I think hong was heading. Correct me if I'm wrong hong.
The Detect spells turn into "Detect My Morals" and "Detect Everyone Else's Morals"
Two ways of looking at it:
1) Two opposing sides. (Absolute Good and Evil, Law and Chaos)
2) Infinite number of conflicting, not necessarily opposing sides. (Moral Relativism)
Both make for interesting gameplay. The only reason why I believe #1 is the default for DnD is because of classes like the Paladin and Monk, the entire alignment system and the way they have the Planes set up.
But if you like #2, go nuts, just make some spells have the "us" descriptor and make the rest of the spells have the "them" descriptor.