His usefulness in combat is really overplayed. The only who's truly irreplaceable is Lae'zel. She's the closest thing to a tank and hits like a truck, while from what little i've seen of him, Astarion may be a bit better than Wyll and SH, but i'd wager homegirl, Gale and any MC could easily overcome anything the EA throws at them regardless of who the 3rd party member is. I know i did, all three times i played through it. And with rogues not being necessary for disarming traps or picking locks it's not like he's needed outside of combat either.
I very much disagree because
With how the games actions are currently set up I think the Thief Rogue is definitely one of the most useful classes (not that I've done any research mind you). Two bonus actions per turn is great, pop a haste potion and get the two ordinactions and you suddenly can make two sneak attacks per turn if you want to (one ranged and one melee).
Sure, it's cheap and probably unintentional. But I will admit to abusing the system and having Astarion carry me through the Gith Fight in my most recent play through.
Yup. A couple short swords, vampire bite, poison, that second bonus action, he's my little problem-solver.
And as for why I would recruit him in the first place, while he's a jackass, it's clear he's a talented jackass, and is very personable once you've bonked him on the head. He seems like a lost puppy, and since we're in the same boat 'pole-wise, I think it's perfectly reasonable that through a combination of pity and admiration of his talent, I'd want him watching my back.