We have several iOS devices in our household (iPod Touches & older iPhones) that all sync through my account. All of them except my iPod Touch are set to manually manage music. However, I'm finding that when syncing, the play counts and ratings assigned by my kids are updating in my account. I thought that the manually manage setting should have prevented this. Is there any way to keep their ratings and plays from showing up in my library and on my device? One of my kids primarily has an iPod Touch for the games and only has about 20 songs on it (which he plays on repeat overnight), which means those songs have play counts in the thousands, while my most played songs are lucky to break into double digits.
I'm pretty sure sure that any time one of your devices is plugged into iTunes it's going to update the play counts regardless of whether or not you have "manually manage music and videos" turned on. More than likely this is because the metadata is completely separate from the fact that you are manually managing the syncing of songs to and from devices.
I'm not 100% sure if that's the case with the metadata but it seems to be given that play counts are updated on your iTunes library even with "manually manage music and videos" enabled.
The only way I could think of checking that would be to enable "manually manage music and videos" on all of your devices and run a test again to see if play counts update. If they're still updating then I would say that the play counts are part of the metadata in the song file that's transferred to and from your library by the devices even when no actual song files are being transferred or synced between them.