Banjo Johnny, don't think of Peter as a Nazi, he's just listing the instruments that have defined the sound. He forgot to officially ban the harmonica but I ran into one situation where having a harmonica player included in a bluegrass show is a good idea... It's when the guy who owns the club where you're booked is a harmonica player and wants to join in on "Roll In My Sweet Baby's Arms." Entertaining the boss can have its merits as well.
It was mentioned earlier in the thread that bluegrass players don't take themselves too seriously but they do take the music seriously. Bluegrass isn't supposed to be loosey-goosey all-join-in good time music. It's supposed to be tightly arranged and precisely performed good time music. The performers work for many years to get their sound and timing just right. It's only naturally that they are disappointed when someone shows up with a twelve-string or an accordian because the clarity of the sound is lost.
It isn't that bluegrassers don't like the other instruments, they do. They enjoy lots of different instruments and kinds of music, they just don't want folks mucking about with their bluegrass.
Peter and I may not agree on the relative importance of each particular instrument but that is the sort of pseudo religious debate that has no conclusion. The point is that, up against the pressures of popular culture, bluegrass is a genre that often has to fight for it's identity.
What would people think at an Irish jam if someone showed up with highland pipes? Or a fuzz-tone electric guitar? My guess is that many participants would politely make excuses and find somewhere else to go. I know I would.
Hope you understand,