Read an interview with Alan Clark, and one reason he thinks The Straits are better than any DS lineup (his words) is the rhythm section, i.e. the drums and bass (Steve Ferrone and Mick Feat). It apparently is very important that the drummer and bassist "click". Maybe both Glen and Mark feel that Ian and Glen is the perfect match? I'd guess musicians at that level would be able to adjust easily to each other, but apparently not.