Hi Larry,
Relatively few users have required the use of an external antenna... maybe one out of every forty or so. Users that have located their iFly low in the panel at the bottom of a stack or mounted to odd metal structure have sometimes needed to use the external, but it is relatively rare. On a few occasions, RF noise from other avionics has created reception problems, and using the external antenna in a more remote location in the aircraft has helped.
Hope this helps, Ed