Delliwin, and the mac alternative DelliMac, are a chimera: tags and folder structure at the same time. If you thought that tags are nice, but missed the more traditional dropbox-style way of accessing your bookmarks when stored in a browser, this is for you. If you are off-line at times, and wish you had your del.icio.us plugins with you, this is a solution. If you never thought there is a point in having your bookmarks in del.icio.us and never used your account much, this may change that behavior.
I’m sure Dario and Shane will write more about the advantages of social bookmarks; in any case, there are resources that you use and are not convenient to keep in the more ‘academic’ format of a reference manager.