I don't understand how "You have to remember to put all your files in a special folder" (Dropbox) is much different than "You have to remember to put all your files on a special drive".
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.
Great app and combined with DropBox, I can sync data folders without having to move them.
NTFS does support symbolic links as of Vista.
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.