Turning off default functions of the OS would constitute circumvention of Apple's OS code, which is actually against the rules of the iPhone SDK, from what I read. So if Apple allows the function to not be present, it is therefore not a part of the OS, but another application that runs over-top of it, which means that you are integrating one app (3rd-party) with another (the iPod app), which requires coding to function right.
We can have this discussion all day, if you'd like.
Because it'd end up being a syncing nightmare?! Having to get the iPod/iPhone to compare and contrast what's already in the device to what's in this second library to ensure it's not syncing duplicate entries of the same track would cause syncing to take significantly longer than it already does, and there's still a large margin of error. The reason it works so well now is that the computer you sync to actually keeps records of the data it syncs and makes note of changes or additions to the library after the last sync is performed so it only makes those alterations the next time the device is plugged in.
This, and I think it's part of the DRM agreement Apple was forced to make with record labels.
Why you would need to sync with other machines is beyond me. Having one media hub is good enough for me, how anyone can stand having their multimedia spread across multiple computers is boggling to me.
But, if you MUST, set it to manually manage music, which will prevent "syncing" but allow for manipulation of the internal iPod's media library to have files to be added from multiple computers (once you tell it to not erase & sync, of course). Huge pain in the ass, if you ask me.