What do you expect to have happen? Sync to multiple itunes libraries? How exactly do you expect that to work?
Besides the technical issues it would be a complete user interface nightmare when dealing with storage issues on the device.
There are a lot of different streaming music services available too(not to mention you can stream your itunes purchases too.) If you are manually managing your music where you expect songs to magically move from one computer to another through an iphone you might consider finding a different solution than complaining that apple doesn't magically solve your problem.
Besides the technical issues it would be a complete user interface nightmare when dealing with storage issues on the device.
There are a lot of different streaming music services available too(not to mention you can stream your itunes purchases too.) If you are manually managing your music where you expect songs to magically move from one computer to another through an iphone you might consider finding a different solution than complaining that apple doesn't magically solve your problem.