SuperSync fills part of that hole by providing synchronization of libraries between iTunes installations, across whatever network connection you have, including the Internet. Providing a remote player, and two-way synchronization is a band-aid on the iTunes problem. It is a good utility at a low price and it gets the job done -- my iTunes library is now seamlessly replicated to my laptop.
But, is that what you really want? I mean, my wife now wants to sync her iPod from her laptop, and she doesn't quite have the storage for the entire library. Now what? New hard drive? And my son... It hardly seems efficient to have a local copy of the library for everyone.
On the other hand, we've gone the server route and that doesn't work either. The wireless access is simply too slow to run iTunes using storage on a server. What's the answer? I'm not exactly sure, but it is pretty clear that for the basic use case iTunes is sufficient, but for situations other than the one person/one library/one computer model, there is some definite room for improvement.