Clefa, Clefairy, and Clefable are all aliens. Fairy is already an unofficial term. Illumigise and Volbeat are light based. Love..... Delacatty, Luvdisc, and many others.
So, I don't think we need new types for all these Pokemon, do we?
This argument doesn't really work, as Steel already had abstract representation via the Magnemite tree, but was made concrete in Gen 2. These pokemon could easily have the new typing added onto them.
That being said, types should be added for the sake of balance, not just for the hell of it, and I'm trying to envision these new types balancing out others, but having a hard time doing so.
For example, Ice could use a defensive boost while Poison could use an offensive boost. I have a hard time justifying Ice's resistance to Light, or Poison's strength against it, so I'm not inclined to add it.