If you needed a handy cutting tool, there are several reasons why you might choose a pair of scissors over, say, a piranha: Scissors, unlike piranhas, are not carnivorous hunter/scavengers; scissors, unlike piranhas, won't attempt to wriggle out of your hand and bite off an appendage; and scissors can conveniently be bought at almost any store, whereas you might have to wade through the Amazon Basin to catch a piranha.
In fact, there are very few reasons why someone would choose the imminent danger of a piranha over the practicality of scissors. Is it bad-ass to make a piranha bite through objects on command? Absolutely. Is it worth the risk of turning your hand into a bloody stump? Debatable. Either way, the man in the video above clutching the piranha must be entirely unfamiliar with the emotion of fear.