They fit the lore as I've said, but I've never been a fan of the south-east Asian style of blade. Too exotic, too flashy.
I don't like axes personally and I couldn't care less about spears and crossbows, but I still want them in since I know lot of other people like them.
NO! Katanas doesn't belong up in the north, damnit (only the blades, which are few now, should have them if they are in, otherwise; NO!)
I think they will be added. The person that is part of the main quest is one of the last Blades. Why wouldn't he have a Katana? Still I find The Japanese culture not really fitting inside ''nordic architecture etc''.
You two need to understand few things.
They are not Japanese, they are Akavir (which translates to Dragon Land - oh the irony), a continent east from Tamriel.
So with you logic, we shouldn't have them in any game at all.
Also, with that logic, an American shouldn't be able to buy French bread or go to Chinese restaurant which is located right across the street (I hope you see where I'm going with this).
The point is, it's nothing strange or wrong to find something form some other culture appear in Skyrim and is actually better this way since it's more realistic and makes things more believable.
Nodachi and katanas will fit Skyrim perfectly no matter how you look at it.