Well, addressing the original point, it seems like this:
* We already have a tool for doing basic cutting/chopping/butchering tasks (stone axe)
* Jorb & Loftar seem to be quite enamored of stone axes (as far as i can tell

)
* A new cutting tool would be pretty much superfluous, unless given an extra ceremonial function (like AnnaC suggests)
* A knife as a combat weapon would effectively be the same as an axe, you'd simply use "slash" instead of "chop" as your attack move (or whatever it is, i don't know)
* Implementing an obsidian knife would require a new forageable object, as said in OP, and it would be a bit silly IMO to add an object like that and have one relatively minor use for it...
So, summing up my opinion, a knife would be neat, and add to the "feel" of the game, but in terms of gameplay it would either need it's own special function apart from stone axes, or it would need to be faster/better than a stone axe, or we would need to come up with at least a few other uses for this new "obsidian" object as a resource. (arrowheads? jewelery?)
"Bein' a minotaur is a lot like bein' a regular human except moo" - J. Rowland