Without a doubt, you can learn much from Zig Ziglar, Anthony Robbins, and Brian Tracy and so you should. What you shouldn't do, however, is try to copy them. The best public speakers are themselves, first and foremost. They are not trying to be or to sound like someone else.
One of the ways you can tell if a public speaker is being natural is to see if they sound the same off stage as do they on stage. Remember the Ted Knight character in the old Mary Tyler Moore Show? As a TV broadcaster, he would drop the pitch of his voice (as well as his chin) when it was time to go on air. Off air, he lifted the pitch of his voice and sounded different than in his broadcasting voice.
[When I was teaching voice to the Graduate Students of Journalism at the University of Western Ontario, I did not show them how to talk in a fake voice, but rather how to find their real one, which was, in most cases, deeper in pitch than their habitual pitch. I did not teach them how to speak in a voice too deep however.]
Individuality in public speaking is what can set you apart from all others: your voice, your mannerisms, your words, your presentation skills - not those of Tony Robbins. His success is entirely dependent on his own personality, his own words and his own style of delivery.
Topics in public speaking are 'a dime a dozen,' meaning your competition is extensive and impressive. Trying to imitate someone else is not going to lead you to success if you are looking for a career in this venue.
Your audience will appreciate you more as a speaker if your individuality shines through versus the personality, the words, the sound, or the style of someone else. They didn't come to hear someone else; they came to hear you.