EKanne - rest assured that when the Claque sing a ballad in harmony it works extremely well. They are all fine ballad singers in their own right. As I think Barry Lister explained, having more than one voice available allows the introduction of some light and shade into something that, with one voice, might be a little hard on the listener. Of course it will not work for all ballads - it certainly serves the Henry V ballad, with it's line,response,line, response format well. I don't think that they would necessarily advocate it in every case -- we were also treated to a superb solo rendition of Thomas the Rhymer from Tom Addison.
By the way, I'd take issue with your implied suggestion that ballads are, per se, long stories. I have one ballad (A version of Henry My Son) that is four very short verses long (short?), another (a very powerful one, The Bloody Gardener, which is only six verses long), and my version of Long Lankin is only seven verses long. Sometimes less can actually be more - and to be truthful I tend to choose the shorter ones to perform much of the time. Ballad forums or workshops do allow the devotee the opportunity to perform some of the longer ones; however, I for one would not dream of performing one of the longer ones in a club or session setting. I can recall more than one session where someone stood up to sing "Tam Lin" and the heart sank - firstly with the knowledge that there would be less singing time for everyone else, and secondly with the knowledge that the singer in question wasn't up to delivering the song, and that you would have to sit there for nearly a quarter of an hour while they proved it!!!
I shall certainly be endeavouring to sign up for another couple of these sessions during the rest of the year .. thanks for organising them Valmai!