But you only know this after you spare her.... There is nothing that tells you she will turn against them if you spare her.
This isn't entirely correct. Yes we don't know what will happen if we try. But Act 1 gives us plenty of evidence of the general ignorance of True Souls. A morally-inclined Tav would very reasonably be interested in trying to persuade or otherwise save them, if able. For reasons both within and without the game, Gut and Ragzlin cannot be budged. Nere has more nuance but is only a realistic option for an evil Tav to attempt to persuade, and it's ultimately ineffectual (now HE would've been a great candidate for an evil path only companion with the currently existing writing). That leaves Minthara who is the least inclined of any of them to try to kill us for simply existing and talking to her, who could always be saved for whatever your reasons might be, and now no longer nonsensically requires the death of the Tieflings to do so. So a Tav with the condition of the True Souls on their mind has their own personal reasons for fucking around and finding out, and sure it's lucky that it can pay off, but whether she joins the party or not isn't the point of this type of reasoning. The reasoning is to try, to create a chance for any we can create one for, and then we find out what we find out.
The only difference between before and after the patch is that trying NOT to kill her while still confronting the Goblins can actually work (without dragging her around as a Sheep and causing a bunch of unintended effects), albeit the restriction on the conditions of knocking her out are still odd.
