Friction, say between two plates, which are bolted or rivited together in a shear application definitly increrases the joint reliability. The key is to keep the joints tight for friction sake and also to promote shear at the fasteners.
Shear, by it's very definition, is to cause the grain structures in the material to shear or displace themselves over adjacent grains or particles.Fracturing tends to pull the grains apart creating less heat therefore reducing the input required to sever the grains or particals from their common bond.
Even though we are discussing the shear strength of screws, it is noteworthy to mention the process of turning in a lathe or milling in a milling machine. Where the geometry of the cutting tool must be so designed that the shearing only takes place at the beginning of the cut. Where once the chip has been sheared, fracturing takes over for the remainder of the cut.
An interesting thaught is to consider shearing metal or even punching/stamping metal. While we call the process shearing, we would wish that we would not have to shear the metal at all. We strive to fracture it. In other words, we set clearance between the punch and the die to limit the amount of shearing and promote fracturing. It takes way more energy to shear than it does to fracture.