Because too often, they grab the tiniest bite before the beak leads the
point back out of the skin/meat whatever, and you end up with a very
precariously hooked fish.

Of course this photo could also be used to illustrate why I NEVER use
multiple hooksets. No way you know if the first set it something like this,
in which case the 2nd set is 99% sure to just rip it free.

http://www.richz.com/fishing/images/no_beaks_for_me.jpg

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www.richz.com/fishing