The AT 4051 mic first mentioned looks like a good choice. It is not an omni directional (this would be bad for stage performance) but it has a wide range - something close to a 2/3 circle around the front of the mic. Just what you'd want to stand a 4-some around - maybe 5.

What I've seen done pretty frequently, is to use this technique for most ensemble play (vocals etc) but when solos come up, the soloist steps off the the side of the circle where a similar mic is set up, free standing, and adjusted for a general standing instument height. Many times, there are two of these mics at either side of eth stage so that the mando player doesn;t have to cross all the way over to the banjo player just to do his/her solo.

The single mic technique is NOT the equlizer you might think. Strong players, strong voices and loud instruments win the day ... the fine individual technologies we've developed to give good sound to every player, no matter how loud their instrument or their playing style is - has vanished. But, as Mark says, there is a self regulating of volume that the group can work - and this in itself can have a nice quality.

The other thing you loose - and this is important to solo singers, is the close mic technique that most solo singers have developed. The mic itself is an instrumnet, an dthe technique you develop can provide a nice advantage ... you loose that ability with the single mic approach.

I think you use this technique for a few reasons; looks good - moves players in and out easily for stage that move multiple bands through an event - allows player most of the control over balance (though frequency and overall volume are still in the hands of the sound man) - most good players can adapt to the 'un-plugged' jam session feel pretty quickly.