Often these kind of songs are telling a sad story, and we all know that the best stories are those that are well told, so practise is I agree very important, but I dont think it is good to totally disassociate yourself from the emotion, and indeed there are times when the fact that the song has clearly moved the singer, has a profound effect on the listener. In Unsung Heroes, the Lost Gardeners Of Heligan, I had to sing two very sad, emotion packed songs, which I had written myself, At the first performance, I disolved in tears at the pure emotion of the thing, which we were performing at Heligan itself. No-one thought this was unprofessional, but more that it was appropriate. I can now sing the song with less of an outburst of sorrow, but still pour emotion into it, to make it felt by the audience if you get my drift.
I was once asked to sing at a sing-around and Pete Collins shouted across the room, Dont ask her to sing, she'll make you cry. I took it as a compliment. Though there are those who have almost made me cry by their choice of a totally inappropriate song or more inappropriate way of singing an emotion packed song. Sensitivity, is I think as important as self control.
I hope you understand what I am trying to say!
Cheers,Lucy