got to chat a little bit with Scott Crump of Stratasys the other day about materials and asked him about fiber reinforced polymers. Nozzle erosion is a minor issue (the big industrial machines have their nozzles replaced after every 3 spools of material anyhow), but the primary problem is just what people have hit on - your inter-layer adhesion (which is already the weak point with a homogenous material) is even more of an issue, as the fibers do not cross between layers.