I'm not trying to get into any pointless internet arguments, but those of you poo-poo'ing the single mic technique sound like you've never done it. You don't need monitors when you are doing single mic, because the band are all playing acoustically 2 feet away from one another. Your ears are the monitors. You'll also see additional mic'ing, sometimes dynamic mics in these scenarios. The typical bluegrass audience does not require ear splitting volume, so it works out just fine. It's not a Metallica show.
The fact is that nothing replicates acoustic tones better than microphones. Pickups, piezo or otherwise do not sound half as good as a well mic'd instrument but are necessary for louder gigs. If you don't need football stadium volume, the mic thing can work just fine.
Just let the bluegrassers have their little festivals and condenser microphones. It's not hurting anyone. Every genre of music doesn't need to have rock n roll fused into it.