Libraries comes "over" the special folder you can still use.
It's allow you to add in the same place, different location (mutliple drive, even network drive/place)
Indeed If you have allready all you data in one place for each, with good organisation, librairies are not so essential, baut it can be usefull.
I have my datas well oganized as you (My Doc, My Musics, etc on separate drive) but as I have a file server I can now access to this location directly from librairies and merge all this data in a same entry point (no more need to add a network drive, only have to add in library).
I use libraries in an other way: I have a folder in "My docs" which is specific to my work (CAO). I created a librarie "CAO", in which I put this folder, and some other like folder with CAO contents: textures, 3D models, etc, , all of that are on a external drive, which is unplugged when not in use => 1 entry point for different place (internal & external drive, network place)
This are some example of what you can do with libraries.
Know that WMP and WMC uses librairies content, so everything you put in there is automaticaly added in WMP/WMC.
About homegroup: It's essentialy a way to make network share easier @ home. Create an homegroup, give the pwd on the other computer, and go.. librairies shared in 2 clics.
For those who are not experienced in the art or networking it's really easy
But at this time it need Seven for all the computer on the network, shame ...