It really is because it causes clutter in the menus and folders and if you have maxed your bookmark number, any useless bookmark is a waste.
Not to mention that the expiry dates make no sense and never have made any sense in EVE terms. There are no sites or sigs that despawn after 2 days. Wormholes always despawn after 24 hours and other sigs and anoms between 4-7 days. The expiry dates make no sense because of that and are useless for bookmarks for these things and most other things that you want to bookmark.