Moral grey areas. The vast, vast, vast majority of NPCs - both on the side of the characters and against them - lie in grey areas rather then absurd black and white/good and evil. It makes for a much richer narrative and much better drama and experience, where choices are made based on the player and the character rather then stock cliches. Note that this isn't just for humanoids - in my current setting, dragons are most assuredly not color coded for alignment, and one of the major antagonistic movers and shakers are a family of gold dragons. Outsiders aren't good or evil either, but typically alien instead. Devils and angels don't differ on "goodness" but in how they view non-immortals; angels feel they should help non-immortals on their own, devils feel the non-immortals need to earn it (thus why devils make pacts), but neither one is unified in wanting to do good or evil. Perhaps ironically, events in the past caused the angels to withdraw - meaning devils now act for and with non-immortals more then the angels.
Is it any surprise I hate alignment?
It also has the added bonus of making actually evil characters seem truly disgusting. If every other orc is a monster, then the general "evilness" is meaningless. If most people in the world are just folks trying to get by, or are doing what they feel is best, or just don't put much thought into it, then having a sociopathic enemy makes them that much more horrifying.
Is it any surprise I hate alignment?

It also has the added bonus of making actually evil characters seem truly disgusting. If every other orc is a monster, then the general "evilness" is meaningless. If most people in the world are just folks trying to get by, or are doing what they feel is best, or just don't put much thought into it, then having a sociopathic enemy makes them that much more horrifying.