How important is the ability to communicate to leadership?  I would have to say it depends.  If you are talking about leading your self, then not too much.  If you are talking about leading your family or a small group then more so, and if you are talking about leading a large group or organization then I would have to say it is crucial.

Starting with this premise, how does the scenario with Moses, Aaron and leading the Israelites out of Egypt work.  Were all the speeches in Exodus attributed to Moses actually made by Aaron?  Did Moses eventually get to the point where he was able to speak himself for God?

One thing is absolutely clear, someone was communicating.  The ability to communicate as a leader is more complicated than simply opening your mouth.  It takes a certain appreciation of multiple factors such as culture, gender, psychology, circumstances, history, nuance, face, etc.  It also takes an appreciation of the power of the spoken language and how a carefully crafted sentence can make all the difference.

Some leaders, especially those who exhibit leadership skills at an early age, seem to be able to intuitively integrate at least some of these factors when speaking.  They may not realize at first just what they are doing but as time goes by, and with some study, they become even more effective.  Someone who I think fits this example is John Maxwell.  He is rightly known as an excellent communicator.  Starting off with natural ability, he applied himself and became a stronger and stronger communicator. 

So, do you have to be a "natural" in order to be a good communicator.  The answer is no, but it will require effort.  Malcolm Gladwell in Outliers describes the process it takes to rise to the top in almost any area, communication included.  In a nutshell, it is simply this, practice, practice, practice.  According to Gladwell there is a direct correlation between the number of hours spent in doing and/or practice and the level of virtuosity achieved.  I think this could easily apply even to John Maxwell, who, even though a naturally gifted speaker, has literally spent thousands of hours in front of an audience of one kind or another.

For those who ever hope to lead larger groups, there needs to be a willingness to take every opportunity, and even make opportunities when possible, to speak in front of an audience.  Couple that with a sincere desire to learn the basics of communication and seek honest critique, and it won't be long before you find yourself improving and leading at a higher level.