This saber is strictly because that hack JJ thought it looked cool. They put zero thought into it before deciding to do it. They then left it to the fight coordinator to make it work no doubt.
I point out that the Jedi have typically been shown to use samuraii styled swordsmanship. The Tsuba similar to a cross guard is not frequently used in that style of fighting. I’d also point out that the cross guard typically keeps the opposing blade from sliding all the way down your blade to your hands.
The blade shown in the underwhelming preview has points near the blade where the light saber cross guard is projected. It has been shown that lightsabers hilts may be sliced by lightsabers. A lightsaber sliding down the claymore lightsaber blade would cut the protrusion projecting the cross guard defeating the main purpose of the guard. This flaw in the weapon design combined with the other difficulties inherent in wielding a weapon as dangerous to the user as these cross guards would be suggests it is not a practical choice.
We have also seen that lightsabers tend to be “sticky” in that the blades don’t slide along one another hence the lack of cross guards in the previous films: they aren’t needed on these weapons.
This lightsaber epitomizes the style over substance I’ve come to expect from JJ.