It really depends on the look you want to achieve. If you want a vast, wide open look you need the wider aperture but you already have an 18mm, which is wider than the 24mm, so you can already achieve that look. If you want a more compressed look, you use the 135mm view. Unless the 24mm lens you are thinking of buying is a fixed lens or a very superior lens, I can see no reason to purchase it other than you just want to.