But Phalar Aluve is a long sword that also happens to be a Finesse weapon. Meaning, you can use Dexterity rather than Strength as your primary stat modifier to use this weapon.
That makes it a great weapon for a Sword Bard, a Dex-based fighter, an elven rogue that happens to have longsword proficiency (though it's not a light weapon, so, you won't be able to dual wield), etc...
This! It's also worth mentioning that You can use
Sneak Attack only with
Finnese weapon which means most rogues will have to use smaller daggers or shortswords, but(!) with Phalar Aluve You can bypass this and also the light weapon requirement for dual wielding with the right build.
My favourite party consisted of two multiclassing rogues(my Tav and Astarion), Karlach(full barb) and Shadowheart(her original cleric domain) or Halsin(full druid) on occasions. From the moment I've got my eyes on this drow sword it's final destination were Astarion's hands. He would have him for all the rest of the game I played even thought I almost never used its singing ability. But thanks to this sword(plus second longsword) and half on half multiclassing Assasin rogue with Battle Master fighter and Two-weapon fighting style, adding also Dual Wielder feat my Astarion often could do sneak attack from hiding, then starting initiative he would do another like five attacks including sneak attack and then next from hiding came my rogue Tav... Thanks to that only +1 longsword any rogue could be a real monster in combat!
Althought I admit that since the sword had a cool cinematic intro, we could get something more from it, like we could meet some Eilistraee cleric or something later who after a nice talk could recognise it and on example give it another small boost. That would've been a nice addition to the story I think.