Can they only speak or can they help you off of the stage? There are a lot of great speakers that can blow you away on stage but fizzle out when you hire them off stage to help your personal business or product.
Some amazing speakers are a lot like actors. They know the material because they have memorized the script. They can wow you, impress you, inspire you and motivate you, but often times, it is the same message and the same talk or sets of talks over and over again.
Just as an actor that plays a doctor is not someone you would want to perform surgery on you, there are many speakers that have learned or copied what works and present it in a working way. But, go spend money with them, (where even some of those speakers have convinced themselves they can help off stage) can become a dangerous and bad investment.