Destemming Kale (or removing the central stem) is really quick and easy, and there is good reason to destem: the central stem is tough and woody, and a bit of a chore to gnaw through even if kale is completely cooked. There are two ways; one is very speedy, and the other takes a more time.
Speedy way (preferred by chefs I know): Hold a leaf by the central stem upside down in your hand. Cupping the thumb and index finger of your free hand around the (now top of the) stem, pull downward in one quick, smooth motion, ripping the tender leaf away from the stem as you go. Check out this YouTube video for a visual
Less speedy, but still good, way: Using a sharp paring knife, cut alongside the central stem until it begins to thin near the top of the leaf.