Over at Remnant of Giants, Deane Galbraith has a good post on Luke knew Matthew, But Still Q.
In The Gospel of the Lord I’ve advocated this and called this view the Holtzmann-Gundry Hypothesis (HGH) to the Synoptic problem (I hope it catches on). It rests on the idea that although Luke used Matthew, Luke and Matthew might have had sources common other than Mark, including some oral and written materials, especially in the double tradition.
Galbraith’s argument is that Luke only uses one source at any one time, so if Luke possessed Mark and Q as sources, one might expect Luke to rely on one source (Matt or Q) but not the other for the double tradition. In which case, it is conceivable that Luke sometimes preferred the source or sources underlying Matthew in preference to Matthew itself. Of course, that would require a case by case breakdown of the double tradition itself to demonstrate.
In conclusion, the dependence of Luke on Matthew does not determine the question of Q’s existence. The Farrer Hypothesis’ compelling challenge to the Two-Document Hypothesis may, rather, be the first step in rehabilitating a reappraised Q.
I think this is an interesting angle worth exploring for those who are, you know, in to that kind of thing!