The Terra Blade is pretty much useless if you look at other weapons.
Horseman's Blade has a higher DPS than the Terra Blade, so that partly kicks it out.
And then the Scourge of the Corruptor travels faster and has higher DPS than the beams.
Eh, I wouldn't be so sure on that.
I've tested both the Terra Blade and the Scourge in the HM Dungeon, and I find that the Terra Blade will mow down whole hallways of enemies, meanwhile the Scourge takes forever to kill two mobs.
The beams pierce 1 enemy for full damage, meanwhile the scourge does damage, and then splits up into two smaller projectiles that seem to do less damage each. Neither of these pierce, but they do home. The homing is nice in some situations, but a lot of times, if you've anything like a hallway in front of you, you can mow a whole row of HM Skeletons down in seconds much faster with the Terra Blade. Also, Paladins are just ridiculously laughable if you use a Terra Blade.
You hit it with the swing of the blade, and the blade beam hits it as it is going through it (and it will hit anything behind it). If you keep walking backwards while swinging, the Paladin will never throw his hammer and it can't hit you either which means you can easily melee it to death
and keep everything behind it dead at the same time, as long as you have a long enough hallway to keep walking away from it. If not, let some wimpy mob hit you and then run through the paladin while you're still immune to lead him back down the other way. Or, simply line a hallway with normal dungeon spikes and have high defense armor so you're constantly taking 1 damage and triggering your damage immunity. Either/or.
Either way, I find the ONLY time I prefer the Scourge is for the guys with the Shadowbeam Staff; the homing projectiles are nicer for taking them down when they keep teleporting everywhere and/or when there are lots of obstructions in my way and no hallways nearby to line up enemies for piercing TB beams.
Meanwhile, the Horseman's Blade forces you to actually strike something in melee to even get its projectiles whatsoever. The pumpkins can really start racking up the damage, BUT, you have to be in melee range before you can start, and it can take the pumpkins a couple seconds to start the damage. There's significant ramp-up, meanwhile Terra Blade's projectiles are there instantly and always.