I think you lose a few things if you make the characters "outsiders". For starters, when the time comes to save the world, our heroes will not really have a personal stake in the conflict. Characters who are not unambiguously good and altruistic might be tempted to wash their hands of the matter.
As a GM, I like to introduce side quests and NPCs from a character's background. As mentioned above, depending on the home realm a character picks, this may not be an option, but the more options in a GM's toolbox, the better, sez I.
Of course, a GM who wants to keep things simple might be better off with characters completely untethered from teh campaign world.