fbmf at [unixtime wrote:1146167638[/unixtime]]But we're not talking about the Forgotten Realms. Why do you have to create a part of the world that a PC won't reasonably ever go to?
You don't, normally. Until it starts interacting with the rest of the world. If Kara-Tur (or any other region) just sorta did its thing and sat there, then nobody cares. Once it's people start meddling in the affairs of the main world where your story focuses on, now it becomes more important to describe it.
Now you dont' necessarily have to do that much world building there, not as far as laying out towns and cities and everything. You dont' need a map of Kara-Tur. However you do need to add a bunch of NPCs that fit into Kara-Tur's influence in the main campaign world.
I mean when a guy makes a samurai spy/scout/whatever, there are going to be quiestions that PC inevitably asks.
The first of which is how he's going to relay the information he discovers. Samurais don't have magic (obviously), so he needs some contact he can talk to who has a sending or teleport spell, or he needs a portal.
If there are portals or teleport magic involved, the situation becomes even more complex, because now the possibiliy of Kara-Tur based help becomes a possibility. So now you've got to work on this master's motivations, and think up a way to fit those motivations into a good plot. You certanly don't want a Kara-Tur army disturbing the balance of power you've set up in your campaign world after all.
Next of course, you need a reasonable way to involve the PC in your campaign. If his master sent him on a quest that involves the focus of the campaign, such as "spy on the zhentarium" then great, that part is done for you. If it's more of a 13th warrior set up, then you're going to have more problems trying to fit him in. First, why does he care? Second how does he meet the other PCs? Third, why do the PCs even trust him? And so on...
Even on a spy mission, you're probably going to need some connection to the other PCs and some reason fro the other PCs to trust the guy. So now you may want to connect the samurai's master with an existing leader in your story. And guess what? Now the DM has to think of why they're working together, what each side gains by the alliance and so on.
Ironically the higher in level everyone is, the easier this becomes. Epic threats naturally cause unlikely alliances to be formed. As for low level threats, one might wonder why Zhentarium smuggling is such a focus to a lord thousands of miles away. So you've got more story to worry about.
Of course, you don't want the lord's involvement to be too major, otherwise he'll be sending in armies and higher level soldiers, and this is supposed to be the PC's story.
I'm not saying this all can't be done, it's just a lot more work on the DM than it seems, and it likely may well be something a DM doesn't want to do. Most cross culture characters that I've seen played have been boring and one dimensional to the point that I give out a groan everytime someone wants to play a wandering samurai.