To play Guy's songs it just would be not fair. Why not to play some Richard's fabulous instrumentals then? Or John's? Or Ruth's? Or someone else's?
To me, if Mark will share a stage with someone (I mean on his solo concert) or share a songwriting credit, the magic of Mark Knopfler will be gone!
As for Mark's opinion on Guy's music goes, I think obviously he likes it. What I'm surprised with, is why Mark didn't have any liner notes written for Guy?