I have a feeling though that Minthara might be the ''hell no, I'm not working with you, kill me or sod off.'' type.
You remember though, that:
- "I do not intend to lead a suicide charge"
- "Goblins are expendable, I am not"
- She spent her whole life "anticipating knife in the back"
- Via mindreading, we get to know that she is pretty cowardly, we can even call her a coward afterwards.
She clearly values her life to a great degree and isn't the most brave person. Also, if defeaten, but not killed:
- She still needs protection from possible drow assasins, as she is an ex-Lolth believer
- Enemies are everywhere, she has no friends as well
- I do not think that Absolute tolerates failure, returning there isn't an option.
At first, she wouldn't join you out of sympathy, to quote Lae'zel - "Practical choice". That would be the reason. She is said to be very intelligent and her possible knowledge of Absolute could be put to a great use for PC. I'm pretty sure she could realise that.
We get to know too, that she isn't exactly the most honest follower of Absolute, probably she sides with them only for power.
EDIT: Grammar and style