Being able to use arbitrary folders is indeed a very useful feature. I'm on Linux so I've just been using symlinks but on Windows I can see it's pretty vital to have it built into the software.
I link my desktop to Zumodrive. So any time I create a new folder or add a file to my desktop, it is available to me on all my computers. Same with my iTunes music. I dont have to move anything any more.
Before the folder linking you had to move or copy studd into a special drive.
Looks like they are targeting it more as a way to share data among a myriad of devices, like PCs and Smartphones, vs. dropbox's more limited focus on just PCs.
Spaceward Ho! 4 (for PC) has this ancient networking code: it communicates with the other players via a file on a network share.