Just my opinion
and frankly sometimes it was better to have the actual person do it because its like when the medical people would say something. It would sound better on camera because you would know that that person has said it in real life so it sounds more real.
Its like in the move friday night lights the people who played the assistant coaches were the actual permian coaches at the time of filming. The director was frustrated with the actors that played the assistants so he had the current coaches from the school play the assistants in the movie and that looked better on film
http://www.pbase.com/bkjansen220
http://www.flickr.com/photos/28877774@N/00

Yes, in most episodes they were, but sometimes the people involved didn't want to recreate what they went though so they used actors.
Green Day Rocks!!! =D