I've had several clients come to me with an "idol" in mind as an example of the kind of speaker they wanted to be. One person flat-out told me he wanted to be like Tony Robbins. Sorry. Not gonna happen.
Another had a more subtle approach. He sent me a video of a president whose speaking style he admires, so I could see one of his role models in action. When we started working together and I gave him some material to read, his tone was a little flat, so I suggested some ways he might add color to his voice. His suggestion was to practice a cadence he's heard this particular president use.
As an experiment in vocal variety, there's nothing wrong with this. And in general, it's okay to have role models when you're starting out as a speaker. Having goals to strive for helps in the learning process; if you don't know what you want to achieve, it's kind of hard to get there.
However, when you decide to model yourself so specifically after a particular speaker, to the point of mimicking speech patterns and cadences, then you're going off track.
Each of us has ways of speaking and expressing ourselves that are unique only to us. When an actor convincingly plays JFK in a movie or a comedian perfectly nails a celebrity's character on Saturday Night Live, it's not just because of good makeup and wigs. It's because the actors have identified and are copying those distinctive speech patterns that make the person recognizable.
The only person who should ever try to duplicate another's speech cadences is an impressionist. It's perfectly all right for Rich Little or Dana Carvey to sound exactly like someone else; it's their job to mimic famous people.
So play with your speech patterns. Go ahead and stretch yourself and see how it feels to slow down certain words, speed up others, vary your volume, adjust your pauses, shrink and expand your vowels, mess around with inflection. These techniques can certainly help you add color and interest to your voice.
Just remember: You are who you are, and there's nothing wrong with that. No one else speaks exactly like you, and that's what makes you special and unique as a speaker. Once you start mimicking the patterns of other speakers, you lose your individuality, and you sound like a copycat! You'll get more respect by being yourself.