If you listen to some of the classic bluegrass recordings, there's an incredible amount of nuance and control in the vocals, and trying to sing along and learn those subtleties is challenging and rewarding. And because there's a strong tradition of harmony in this music, the sustained vowel sounds Eno talks about are everywhere.
While the group singing Eno talks about is generally frowned upon in bluegrass, nailing a close harmony stack with one to three other people (depending on the parts) is a fantastic feeling. Building relationships around that, where you're looking forward to the next festival or music camp because you'll get to sing with someone again, is even better.
I got into this music for playing my instrument with others, but while I still enjoy that, I appreciate the singing a lot more now.