Now that it's confirmed, not sure I see the point of the Legendary change on PWs either.
It gives them leeway in printing more versions of the same Planeswalker since now it's no longer, for example, Liliana, the Last Hope OR LotV as to what you can include in your 60 or 100 card list. Superfriends decks can now include different versions of the same Walker without worrying about drawing copies of different versions. Maybe it even helps them push into new design space since they won't have to worry about crowding out all other versions of Jace (or even being crowded out from the start). I'm sure there's some desire in the casual crowd to be able to play decks full of their favorite Planeswalkers. Then there's the streamlining effect it has on the rules themselves that probably makes it easier to grasp for new players.