They're not elemental damage, they're pure piercing. They just happen to trade out some of that piercing damage for a status, much like pretty much every other status-inflicting weapon out there (Plague Needle, the status alchemers, the haze bombs) other than Brandishes, which are widely agreed to be kinda unbalanced as they are. I have absolutely no desire to see something as ridiculous as a Shock Flourish, either.

We don't really need that much curse. It's meant to be a strong but uncommon status with penalties to using it, hence why the vials are so rare. I wouldn't mind adding it to a gun *cough*GrimIronSlugremake*cough*, but having lots of weapons inflicting curse...
I'll pass.

Sealed Swords are common in t2 because they do a ton of normal damage, enough to compensate for the fact that the damage dealt is reduced by pretty much any armour in the game. People don't use the charge, the charge doesn't inflict statuses with enough frequency to be useful in LD, and Curse isn't one of the statuses that the Sealed can inflict. In addition, the reason Faust and Gran Faust are used is because shadow resistance is relatively uncommon in t3 LD, and the normal swings do a lot of damage even without factoring in the potential Curse ability. Divine Avenger does the same amount of damage, except split normal/elemental. However, since Vog tends to be a bit more common than Snarbolax, gunners generally seem to use split PMoS/Shadowsun, non-Recon bombers have no other option than elemental defence, and defensive armour is pretty uncommon in t3 LD, the Divine Avenger generally gets less damage output than the Gran Faust.