I do most of my work on my Windows machine. Since Spine projects (and their images) are pretty small, I just put them in my Dropbox folder so I can get the benefit of a little bit of backup/versioning + I can work on animations on my laptop.
Incidentally, my laptop is a MacBook running OS X.
I assume the .spine just stores the string when storing the export path.
But as you can imagine, a Windows directory/file path makes no sense in OSX. And, I assume, vice versa.
I don't think it's a huge deal leaving it unhandled vs. having the project be aware that the stored path was for a specific OS/specific machine. I think it's just something people should be aware of. It's not like it causes Spine to crash or prevents work from being done.
[EDIT] Oh.. but when I click "export" in OSX with the Windows path still in the textbox, the export progress bar comes out, fills up without hitches, and Spine says "JSON export complete."
I wonder where that file ended up though. xD