Do you know what stalwart Bluegrasser has NEVER followed the "D-28, F-5, Mastertone" trend? Mac Wiseman. Saw him in the late sixties at a Monroe festival, using a Gibson J-45!!! Talk about heresy. He got ribbed a lot by the other (pro) pickers. He really broke ranks when he started playing Yamahas later on.
On the other hand, if a band wants to be taken seriously "within" the Bluegrass hierarchy, and not have to buck the common wisdom, the "big three" instrument models help. I love Bluegrass, but it would be hard to find a musical form that is more rooted in tradition. Yup it's "narrow", but what are ya gonna do?