I still think in any morally ambiguous system a 'trustworthiness' system works best. If you're really evil but always keep your word, that should count for more in a 'morally neutral' world than doing the 'right' thing.
We're really talking about an entirely different way of measuring ethics, after all.
If we're talking about a Thieves World type game (Very famous series of short story collections. I'm sure most of you have heard of them) then we're not talking 'good' and 'evil' anyway - just 'evil' and 'more evil'.
But even the bad guys keep track of who they can trust.
In a morally neutral world, it's also a far easier thing to quantify. You either keep your word or you don't, after all.
