I've been wondering the same thing.
I have MC 15 installed in 'portable' mode right on my device. Every time I open it, it starts saying "Saving tag changes" in the status bar, and if I watch the file system activity with SysInternals Process Monitor, it starts updating a lot of files on the device. This in turn causes the device to rebuild its internal library when I unplug it, since it detects all these files changed. Since I have gigs of music on here, it means my device is unusable for several minutes, every time i unplug it from USB.