It's partially due to the fact that you lose out on THREE companions and the ENTIRE Tiefling story and willingly choose to have your character go down an evil route only to realise the companion you wanted to recruit is actually NOT EVIL IN THE SLEIGHTEST that makes your statement completely false. It would've been alright if they made her to actually work with an evil playthrough but when you save her and free her from the Absolutes control she presses you and asks you why you helped her kill all those people seeing as she was forced to kill them against her will and you willingly chose to kill those innocents. If you broke her from the mind control in ACT 1 and asked her if she still wanted to attack the grove she would've been fully against that idea and would've likely left camp had you gone through with it which means theres no actual reason to lock her behind an evil only playthrough. So either the completely change her character so she doesn't ask you why you decided to slaughter innocent people or they actually give us more options to recruit her so we arent forced to slaughter the entire grove and lose multiple companions.
Minthara is definitely evil. But she's pragmatic evil, which is why it doesn't mesh well to have her recruitment locked behind the senseless slaughter of innocents.
