You can make do without an axe if you have a good knife but I find some tasks to be easier with the "correct" tool and therefore use less expended energy in a survival situation. The biggest difference I have found in tasks is in cutting a slash through bark in order to set up a drain for sap. Much easier with an axe when the diameter of the trunk increases. Becomes very difficult on a large tree with a knife. Although maybe my technique is just poor. I own one of those hatchets and although a bit small for my purposes they are fine as I'm not going to be felling any large trees. I also bought one for my girlfriend. She finds its the first hatchet she has ever considered carrying since the weight penalty for an axe was always too much for her.