We've called one to a scene, at the request of a patients family. But just like everyone else, no visitors in the back while enroute (unless someone is on deaths doorstep-and doesn't want any help from us to prevent that). I don't and I won't pray with patients, it makes me uncomfortable. I'll gladly hold their hand, while they do it.
I'm not a religious person, in fact I get very pissed when people try to press their beliefs upon me. I use the box as a neutral, judgment free environment. If we have people afraid to talk, because of someone listening in, be it a cop, a fellow employee, the press or a priest.. It may damage what little safe haven of trust, that may exist in the back of the ambulance.