As long as your hands aren't completely filthy and you don't eat right over your MacBook, you don't really need one.
I don't need it either, 3 different MacBooks over the last few years and not once I found my self in a situation where I needed it.
But it's nice to have though, that combined with the outer shell can ensure that your MacBook remains in a near flawless condition, which can help when/if you decide to sell it.