How does this sort of silliness make for a good story?
It doesn't. This is the part where you, the discerning GM say "You are obviously being ridiculous, and beyond the novelty this will not be fun for anyone. You can't do that." but just because abuse is
potentially possible does not mean that we should scrap something that could make the game completely awesome. Consider the following scenario.
The wizard throws his last bit of juice at the villain and the villain is thrown. He stands there, broken and bleeding waiting for the end to come, but it doesn't. Finally he stands, laughing. "You've got nothing left do you?" He says. He starts limping away as he laughs, and the player goes "Can I dredge up my last ounce of strength and throw something at him?" You decide that it's ok this time and the wizard throws one last blast, knocking the villain off his feet to land nearby. The wizard sees the last of his enemy's life drain from his face as he begins to drift off. He hears footsteps and a deep unfamiliar voice says "Well, look at what we have here." Then blackness.
This is the one thing that I love about pen and paper role-playing games. In a video game or any other sort of interactive media you must have the rules set, because there's no way for it to adapt as you come up with interesting ways to work them. However with pen and paper RPGs there's another human being on the other end. One who can make decisions based on what is fun or exciting. One who can create scenarios like I outlined and yet still not allow the abuse of that particular "rule" if you will. I don't intend to be inflammatory, however it seems like if any GM is allowing such abuses (or sees them as a real threat to their game) then they aren't doing their job. One is there to be that balancing factor, to decide what will be great and what will not. A good GM makes decisions based on what's best for the table, not what the rules state and not even what you have said in the past.