There have certainly been some venues I've played (house concerts, coffee houses, etc) where I have not needed to use sound reinforcement. But just as certainly, I have played at festivals and concert halls where the paying audience would not have been able to hear and enjoy the music without a good sound system and a competent sound engineer to make sure every act can be heard throughout the venue.

Concert and Festival promoters must provide adequate sound reinforcement for their event. Maybe a folk purist would like to have a totally acoustic community, but the "bottom line" is that if they can't hear it well, the audience will not attend.