I am a pretty hardcore fan of Tolkien's original works - and I have to say - writing Legolas in isn't that huge of a departure or that infeasible in the continuity of the events of the Hobbit. Bilbo goes to the halls of Thranduil for goodness sake.
Several of Peter Jackson's alterations to The Lord of the Rings (e.g. Arwen, Osgiliath, Gandalf's confrontation with the Witch King, the ridiculousness that Treebeard wouldn't know what Saruman was up to in his own forest, etc., etc.) were much more inflammatory than this and, even so, I'll be the first to argue that Mr. Jackson has handled
enough of the content from the books with deference and respect to warrant a few liberties like this here and there.
Still, all will be forgiven if he includes Beorn