Splitting wood with a knife is safer, and more effective and controllable for making kindling etc. Think of this...Swing the blade through the air, and hope to hit your mark (without splitting your foot, or having the axe head fly across camp), or swing a piece of wood against the blade that sits firmly ontop of the exact area you intend to split. Hatchets are for chopping and axes are huge.
My bk7 was able to go through logs that I would have had to chop into sideways with an axe, and a hatchet would have been a joke. If you're processing wood for the winter season, and just need to take big, cut logs down to firewood size en masse, Axe all the way (I love axes when I can afford the weight), but if you're making campfires, and really only need to split a couple large logs down, and make kindling, I would prefer a strong, solid knife. A BK2 works essentially like a splitting wedge, you could shingle a shelter, make flat, even food boards and then cut up your food with the same tool, all kinds of stuff you simply cannot do with an axe.