Yeah her recruitment is unnecessarily obscure, even a massive disrespect to the cast member by being flaunted as nothing but mere unfinished compensation for deviating from their intended story path. But not really metagamey because the player has plenty of story reasons to approach, interact and join with Minthara, as recruting her legitimately by raiding the Grove is just a regular story choice of good vs evil.

KO recruitment on the other hand used to be extremely metagamey only because it got poorly implemented, as she needed to be temporarily hostile within the Goblin Camp for it to work. But today it is fixed, so it involves zero metagaming to recruit her since it is the same form of recruitment as Minsc's by just bonking them on the head.

My first ever playthrough came with a natural reaction to knock out every True Soul, since the story in ACT 1 quickly establishes that they are just victims mind controlled against their own will, so I intentionally tried to avoid killing them by knocking them all out. Naturally it led nowhere back then because Larian had no such implementations in place, but there are lore reasons to do so despite the game not saying to do it, if people pay attention to what's happening in the story.

There is even an Acolyte at Moonrise Tower Docks who has an entire scene built around this dilemma, where the player can completely break the tadpole's influence over the poor man and then he ends up panicking so much once he becomes aware of the parasite in his head.

So there are plenty of lore reasons not to be murdering True Souls as long as the player's paying attention to the story, since they're just victims. But naturally those who don't will end up missing out. Many people also lost out on Minsc too that way because they weren't paying attention to Jaheira pleading not to kill him.

As for the question assuming it's not rhetorical; yes, how she is recruited makes a huge difference, not for her character but game's overall content.

  • Recruiting her legitimately by raiding the Grove kills the content of over 60 NPCS, several companions and takes away Shadowheart's romance... but enables the player to have sex with Minthara's dominated body (I say body intentionally here because that is not Minthara in ACT I).
  • Knocking her out instead takes away her one and only camp scene in the entire game, but saves the content of over 60 NPCs and all companions so ACTs II and III are significantly more fulfilled.
  • Outside of that Minthara herself reacts just a tiny bit differently in ACT II depending on her recruitment, but that's about it (I don't mean just the post-Ketheric dialogue, but also when entering her mind while she's being erased).