What do Orc bandits do with the loot they take from merchant caravans? What are Goblins planning to buy with the 1-8 silver pieces (of legal tender in human settlements) they carry in their purse. How (and why) do all Elves learn to speak Gnoll and Hobgoblin (without speaking to any Gnolls or Hobgoblins)?
They are the LAW! (Iain McCaig, from City of Thieves)
In my fantasy games I have a habit of introducing a hefty dose of interaction between humans (and demi-humans) and the races of evil humanoids. This might well be the product of a thorough steeping of my youthful imagination in Blacksand, though little things such as the fact that Ogres seem to be a not uncommon feature of human settlements in the Old World of Warhammer Fantasy Roleplay, and that there is an Orcish ghetto in Alfheim in Mystara's Known World has undoubtedly leaked into that stew, too.
So, what is the degree of human, demi-human, and humanoid relations in your games?