Role models are hugely important. Think about all the people you've heard speak, and then think of the ones you admired and who made an impact on you. Perhaps you had a particularly charismatic teacher. Maybe you've gone out of your way to find sources of inspiration, you'll probably find it difficult to name more than a few. Religious evangelists and personal development speakers may have well-developed speaking skills, but it's hard to think of how to translate those skills to the business or technical presentations you need to make.
If you don't know what a great speech sounds like and feels like, you'll find it difficult to improve your skills and to help others improve theirs.
If you don't know what a great speech is like, you may be able to polish the mechanics of your delivery, but you'll miss out on deeper opportunities to improve your public speaking. You can give a good speech without ums and ahs, with vocal variety and body language, and with good eye contact. A great speech, however, shows you how all those things fit together with great content, great organization, and all the other factors that make a speech extraordinary.
If you don't know what a great speech is like, you'll be able to offer only surface suggestions to other people interested in improving their public speaking skills. You can help them eliminate the ums and ahs, encourage them to speak more slowly or quickly, and help them explore vocal variety. But you'll find it difficult to recognize their key strengths and help them imagine how they could do it even better, and you'll find it difficult to make specific suggestions that can help them transform the way they communicate.
Why limit yourself to that, when you can find tons of inspiring speeches on the Net?
The key resource I recommend to people who are interested in improving their speaking skills is the Technology, Entertainment and Design conference, which shares speeches from some of the most accomplished people in the world.
You can also check YouTube and other sources for comedians, poets, politicians, and other people who make a living--and make a difference--with the spoken word.
Go find yourself a few role models, and see what a difference it can make. =)