The crossguard idea purports to solve the problem of an opposing lightsaber sliding down and hitting the defender's hand. Some, like The Verge's T.C. Sottek, argue that even if this is a concern, the way it's been implemented here doesn't actually protect the hand.
As anyone who's watched the Star Wars movies knows, lightsabers can cut through pretty much anything — including other lightsabers, as when Obi-Wan Kenobi sliced Darth Maul's double lightsaber in half. So the fact that the crossguard saber leaves a bit of room for an enemy lightsaber to come slicing through the emitter seems to defeat the whole point of the crossguard. And that's silly.
But hang on. There just may be an explanation for this that makes the crossguard a little less implausible than it might seem.
In the Star Wars universe, there are certain rare materials that are thought to be lightsaber-resistant. Among these are substances like phrik, cortosis and Mandalorian iron, which have been used to create weapons and armor that can deflect or short-circuit lightsaber beams. There's a precedent for lightsabers made of phrik: Emperor Palpatine's lightsaber was, according to Star Wars: Revenge of the Sith: The Visual Dictionary. In Star Wars video games, cortosis has been shown to block lightsabers, and sometimes shut them down.
Maybe J.J. Abrams has really done his homework on Star Wars technology. Maybe what he's telling us is that this Sith saber is made of one of these resistant materials.
Okay, probably not. It's more likely the producers just made a silly design choice. And of course, even if the crossguard emitters were made of cortosis, why not just make the whole crossguard out of it, too? Seems like that would be easier.
But until I'm proven otherwise, I'm going to assume the Sith have figured out how to manufacture lightsabers with superpowers.