I think the logic depends on one`s personal point of view. I, for example, never really cared about Queen`s live recordings so much (I only bought the live albums to complete my collection, though I still listen to them sometimes). Therefore I see the studio versions as the definitive recordings of all the Queen songs. So in my opinion 39` would only have been properly presented if it would have been sung by Brian (like in the original). My logic may however not work for other people who appreciate Freddies interpretation.
But still they should have interpreted one or two Queen songs at the concert with the "original" voice on it (since there aren`t many who Freddie didn`t do it could have been only 39`by Brian and / or "I`m in Love With My Car" by Roger - the others are even less known / like "Drowse" etc.).
Peter, now and again, as me and Sebastian wrote - it wasn't solo concert of Brian! I see your point, but why you can't understand they invited friends to sing their songs? It was tribute to Freddie, not to Queen! I'd like if tribute could be played by Roger, John and Brian and nothing else, but if they choose another way and asked guys to sing their songs - I'd accept it and don't wonder why Brian didn't sing it! Brian plays harp on LOML on album, but he didn't has to play it on live shows!