So, let's say you've somehow stuck a deer. You're lashing seperates, or the shaft breaks, both of which are not unheard of with field expident spears. The lanyard is around your wrist when the deer takes off.

One of two things happens. One, the lanyard slips off your wrist, and you have a hand free to wave byebye to your blade or flip off the animal. Two, it tightens, and a 100 to 150 pound animal that has just gone from 0 to 30 pulls on it. Your arm really doesn't like that type of force, with luck your shoulder dislocates rather than breaking something. (You can relocate your own shoulder, you really shouldn't try to set your own arm.)

Or if you have your lanyard attached to you through some other means, you get a varient on the nantucket sleigh ride until something fails. Sure, the blade may fall out, but your time and energy have been waisted, your equipment and you have probably been damaged, and you still have no meat, unless you are a good tracker. But you have inflicted suffering.

Plan for the worst case, not the best. In my childhood, I made a number of knife spears and used them on the targets. You'd be suprised at how badly it works unless you are using a very good shaft and a purpose made head.

For short term survival, you are better off using a light spear for the a coup de grace, or as a grabber of fruit, than for hunting. Your best spearing is going to be frogs and fish, at night with a torch. If you want to get something like a woodchuck out of it's hole (yes, the do live outside suburbia, but you'd have to be very hungry) is with a forked stick snarled in it's fur.