While many pro Bluegrass bands perform with one or two large diaphram condensers, almost none record this way. Ricky Scaggs did it on the Bluegrass Rules CD, but hasn't done it since. I think the artists and producers probably want more contol than the single mic allows.
Also, if the band is not top notch, you would probably like the opportunity to fix some spots.
It is my personal belief that many bands play live with two mics for 1) it gives more movement on stage, which many feel is more interesting to watch and 2) it takes the bad PA operator out of the loop ("just set it and leave it!!"). Having played Bluegrass for years, there are SO many sound people who have no clue about live acoustic music sound.