You raise good points, it does take a certain personality type to be successful as an evangelist.
For me, curiousity is the driver. It is sometimes tricky to balance learning, presenting, etc...however, you should note that we often present the same information multiple times (and sometimes in multiple formats - i.e. live, web casts).
The other thing you mentioned is also important - perfectionists will be challenged by this job.
Those who have a certain tolerance for ambiguity will probably be happier in the job - between working with beta software and constant change, flexibility is a key trait.
