Whoa, some people are going to die if it ever happens.
First rule of hiking/camping. PREPARE! The 6 P's; Proper Planning Prevents Piss Poor Performance. If you're going out into an area like that you get malaria shots, take the common anti-venom, adequate First Aid kit, make sure you have some kind of decent comm's equipment and suitable clothing.
The cutting the hand off option in the case of a snake bite/spider bite would have to be quick. ACK! CHOP. Or aim for the elbow. A better option is to get a pressure bandage on the limb, splint it, apply an antivenom and call for help. Sucking it out will only kill you more surely.
Cutting your hand/arm/whatever off if stuck or trapped is another thing again.