Why bother with the code? The value is with the existing links database, and since it belongs to the users, Yahoo shouldn't have the right to sell or distribute it freely.
Since users can already import/export their bookmarks, the only support Yahoo needs to provide is keeping the import API open for a little longer after the site is shut down. Of course this assumes that users would want to migrate their data to a replacement service.
Because they wanted to research how people's bookmarking activity corresponded to new information emerging concerning the 2009 flu pandemic http://en.wikipedia.org/wiki/2009_flu_pandemic
Unless I am mistaken here (as far as copyright goes, if that's what you're talking about) the only data Delicious' users can claim any right to is the short descriptions they are able to add to each bookmark. The mere fact that they liked that page is data that can be freely distributed.
Since users can already import/export their bookmarks, the only support Yahoo needs to provide is keeping the import API open for a little longer after the site is shut down. Of course this assumes that users would want to migrate their data to a replacement service.