I always hated how stupid it makes adding a few songs to an existing device. Half the time it wants to re-sync everything and wipe the device while doing it.
No, just ill-informed. You can re-download your purchases (which are DRM free) from the iTunes store on to your device. There is absolutely no need to use iTunes to manage an iOS based device. Not for registration, backing up or adding media.
Make it too cumbersome for your users to put an mp3 from their pc on their iPhone and they might just buy the song again from iTunes ?