They've established that light saber "blades" aren't just laser beams, but focused energy constructs. That's why they have a finite length, and why they cannot pass through each other. In a practical sense, it's because Lucas wanted the fights to be like sword fights visually, but most people have figured that part out. But the part that never worked for me was the lack of hilts, and now they've addressed that. Those little guys on the hilts will stop the opponent's blade from sliding down and taking off your hand because their blade will stop. We have seen the blades sliding up and off of each other; it's not too great a stretch to figure they go the other way.