i use the gerber LST ultralight 2" locking blade. it weighs .5 ounce. and its cheap. they have a bigger version that is only a couple of ounces.
while i agree that it obviously would have limitations it still can perform the following duties: cutting wood, creating a spear head, cutting bandages, etc.-- all of which address issues that people advocating a heavier blade cite.
to cut wood you just have to have another piece of wood or rock to use as a hammer but it does work. as a spear head consider the obsidian spear tips native americans used... they arent much bigger than the LST. lash that onto your hiking pole and there you go.
someone above said they carry a bigger knife in the event they have to fight off a bear. good luck with that. you must like that last scene from legends of the fall or something! if your using your knife to eat food with, then youre keeping that on you at night for protection, your asking for a bear problem that probably would never have occurred but for the knife!
i think a knife would be useful if you were attacked by a mountain lion, as they dont have that thick of a hide, whereas bears obviously do. and a lion, once they get cut, will probably realize your not prey and let go. if were talking items to be used as weapons, i think a hiking pole a much better in the backcountry than a knife; it has a much longer reach, a sharp point, etc.