I will use wedges to get my machete out when splitting nasty wood. Sometimes it gets stuck in due to knots, and you can't get it out. It is not too bad to fashion a point on something and drive it in behind the blade to loosen it up.
You will often make a several different size wedges on the spot to do the job.
It is amazing how much holding power knots can have. They are the reason people end up with the misshapen poll on their axes, from trying to drive the axe through the knot. When you get into that kind of wood, you really want to work smarter, rather than just beat on your tools.
I think in tough, knotty wood, knives with a heavy grind and thick edge would fare better, because they work like a wedge, and keep the wood away from the edge once they are actually splitting. I don't know if that is a great excuse for carrying an extra pound of steel around though.
ETA: Back on topic, my wedges often don't look like wedges. They may look like crude wooden tentstakes, or whatever. Whatever you need to get the job done. I have heard of guys makeing a few out of hardwood at home and carrying them around. They could be cut up for firestarting if needed as well.