The slingshot as a defensive weapon

What becomes clear in the book and both film adaptations is that the wrist rocket SLINGHOT is the unsung hero. Ben finds the dead prospector's slingshot, marble ammunition, practices using it, and therefore has a projectile weapon to ambush the stalking Madec. It would have been much harder for a weakened Ben to attempt to physically ambush Madec.

So the question is, is a SLINGSHOT a viable weapon for self-defense?

I'll give you the qualifying answer from the question, "viable".

Yes, a slingshot can be a, "viable" self-defense weapon. That does not make it an optimal self-defense weapon.

What is the specific need?
1) For non-lethal self-defense, a stout, 3-foot, 4-foot, 6-foot staff and the knowledge of Asian or European stick or staff fighting will suffice, even against attackers armed with non-firearm, non-projectile, hand weapons.
2) For lethal self-defense against a firearm-armed assailant(s) or multiple, armed attackers, nothing beats a good pistol or shotgun. Carbines and rifles can be used but are not as handy in close-range situations.

Yet, read articles on the Internet and the slingshot is held to be a good companion for hikers, campers, hunters, and outdoorspeople. One article's statement made good sense. Brandishing a slingshot, ready to shoot, might deter a possible attacker by showing him that you are not unarmed. Certainly armed with a slingshot gives you good stand-off defense against what could be an overwhelming adversary, that is, a much bigger, threatening, hostile, possibly deranged fiend armed with a knife and coveting your personal belongings, money, and/or your girlfriend/wife. Hand-to-hand self-defense is great, don't get me wrong, but it still entails you coming to grips with an assailant who might be unhinged, drugged up, and especially armed with a hand weapon, most likely a knife, if he wasn't wielding a firearm to begin with. I'd rather keep an assailant at bay with a slingshot rather than risk a grappling confrontation, at first. Hand-to-hand self-defense should always be the last option if there is no escape or no way out, because the risk of being severely injured or killed is still high, especially if your attacker is armed.

And why a slingshot? It's small, lightweight, and portable, and lethal.
Let's put it this way, in most states you cannot carry a pistol publicly so even if you own a quality semi-automatic 9mm or .45, it's going to stay in the drawer at home while you're out and about in the forest hiking, camping, fishing, or hunting.

CAUTION DISCLAIMER: Check your local and state laws regarding the legality of owning and using a slingshot. Some states like, Massachusetts, New York, and New Jersey, prohibit slingshot ownership. It is a crime to own a slingshot there. As always, you bear legal responsibility in any kind of situation where you may find yourself using a slingshot for self-defense. Misuse of a slingshot can also bring legal punishment. Slingshots are not a toy. Slingshots are weapons. There, I had to say all of this.