No, the target device has no bearing on the file that KindlePreviewer ultimately creates. It, and the command-line kindlegen both build the joint MOBI/KF8 exclusively, now. Thanks for the wiki pointer, but I have a fairly decent grasp of MOBI/KF8 and the PRC container details.
I was just pointing out that while you're right -- the mobi format itself makes no use of the NCX file past conversion -- users are no longer able to create an ebook in "just the mobi format" with Amazon's latest toolset. Hence: they may as well get used to the idea that NCX files are going to be increasingly "used" by devices that utilize
kindle/amazon formats.