The problem with the cross-guard lightsaber, from my thinking, is that it highlights a problem lightsabers have always had: if I slide my lightsaber down your lightsaber, I will cut off your hand. We can argue that, like the katanas they're based on, the fighting style employed by lightsabers prevents that from happening, except the prequels have shown us that, given the choice, lightsabers are not used anything like katanas.
That being said, the kind of rapid-swing, choreographed fighting style exhibited in the prequels would prevent me from sliding my lightsaber down yours and cutting off your hand, but that just brings us back to the cross-guard lightsaber:
What possible fighting style could you have where that's effective? Watch this video: https://www.youtube.com/watch?v=5hlIUrd7d1Q
They start using the swords at around 1:50. That's the traditional way to hold a sword of that size: one hand on the hilt, one hand on the blade. I'll give you a moment to think about that with a lightsaber.
Clearly, that's not what we're talking about. And, given a lightsaber's weight's going to be all-hilt and not that heavy no matter how long the blade, let's start talking about light, one-handed swords, even if that's clearly not what this sword is patterned on.
It's always seemed to me that lightsabers should be employed as fencing sabers. They're light. They move quickly. They have substantial reach without unbalancing the weapon. But fencing swords must have handguards because, in fencing, without a handguard you lose your hand. Even with a handguard, you're likely to take several hits to the arm before anything else.
So, let's assume this... light-claymore is going to be used like a fencer. I doubt it; I bet that sith swings the sword like he was Ned Stark the gymnast. But let's say he handles it like a fencer. The thing about those little side blades is... it looks like an opponent could just cut through the emitters, rendering them useless.
Oh, they're supposed to be offensive, not defensive? I would like you to think about getting the hilt of a sword close enough to your enemy and 1. you have not already stabbed them or B. they have not already stabbed you.
Now, if the Colbert Defense turns out to be true (which I doubt; I'm fairly certain one of those light-nubs is going to be cut off in a climactic moment), then maybe we have something here. Maybe we had a sith who looked at a lightsaber and said, "No wonder there's so many sith and jedi missing hands," and decided to do something about it. This could turn out to be an interesting weapon.
But I'm pretty sure someone said, "You know what would be badass? Lightsaber cross-guard," and they didn't take it any further than that.
As an aside, what's the limiting factor on the length of a lightsaber? Power? Focusing crystal quality? Because it seems to me, whoever has the longest lightsaber has a clear advantage. If I can stab you when you're fifteen feet away, you'll never get far enough inside my guard to hit me with a traditionally lengthed 'saber. Clearly, we can't do light-polearms (an opponent would just cut through the pole), but if the blade of my saber is twelve feet long, apart from collateral damage (which, who cares), there's not really a downside since the blade is weightless.