Some say she stole them in order to wean Lavan off of idolatory (e.g. Rashi).
Ibn Ezra asks a kasha on this:
That teraphim are idols can be ascertained by Laban’s referring to
them as gods (v. 30). Some say that Rachel stole the teraphim in
order to keep her father from idolatry. If this were the case, why
did she take them with her and not bury them on the way?
The most likely reason that Rachel stole the teraphim was that Laban,
her father, was an astrologer, and Rachel feared that he would look at
the stars and discover which way they fled.
Radak says on this:
According to Ibn Ezra each tablet had the face of a human being and was presumed to get inspiration from celestial regions. Rachel’s objective in stealing the Teraphim was to deny Lavan knowledge about the route Yaakov had taken when he left.
So, according to this, she stole them in order to prevent Lavan from using them to find Yaacov.
However, there is still a case to be made about "weaning off idolatory", from Rabbeinu Chananel:
ותגנב רחל את התרפים אשר לאביה; she stole them in order that he would reconsider his actions, saying to himself that any deity which allows itself to be stolen surely cannot be much good to anyone. The same argument had been used by Yoash who said if baal was really capable of avenging himself, he should do so himself instead of letting his human worshippers become his defender (Judges 6,31). Similar arguments are reflected in Ezekiel 28,9 where the prophet predicts the downfall of the King of Tzor (Tyre) who had declared himself a god. He ridicules this “god” as saying to his murderer: “I am a god!” (quoted by Rabbeinu Bachya).
It seems that he is saying that if they were destroyed, then fine - there are ways of accomplishing that would have made sense to Lavan. But them being stolen is something that would make him realise that they are powerless.
Rav Hirsch also emphasises this by saying "they had to make it clear to Lavan they were powerless".
This still doesn't answer why she didn't have an obligation to destroy/bury them. An answer brought by Ramban is that they were not necessarily only idols. Tur HaAroch says:
Ramban admits that it is possible that Lavan had been using the
teraphim in his worship of idolatry. It is however by no means certain
that teraphim served only as idols. We certainly would not expect to
come across idols in King David’s residence, and yet we are told in
Samuel I 19,13 that Michal, David’s wife placed such teraphim in
David’s bed, feigning that it was he who was sleeping in that bed.
Surely, David did not keep idolatrous figures in his home. It is most
likely that the teraphim were objects which enabled people to know the
time of day, and in that connection they were also used to help them
to predict future events...
She likely wasn't under obligation to destroy them as a result, which helps fill in the holes left by the previous answers. They were "inaccurate prediction devices, similar to clocks", and therefore they are permissible to use by someone who knows they have no power of their own and puts her trust in Hashem.
However, he goes on to argue against the above explanations:
...Some commentators believe that Rachel stole the teraphim in order to
wean her father from practicing idolatry. If that were so indeed, why
did she keep these teraphim with her instead of at least burying them?
It is far more likely that Rachel, being aware that her father was an
astrologer, was afraid that by a combination of astrology and the
teraphim he would succeed in tracking the movements of Yaakov and his
family and he would overtake them in short order. Another view is that
she took the teraphim so that these would not reveal to him where
Yaakov was at that time.
Surprisingly, he seems to be saying that she was simply denying him use of them, rather than trying to prove any sort of point. I would imagine she didn't have any obligation to destroy them, so the quickest thing to do in that shaas hadchak was to shove them into a bag and run.
Pirkei D'Rebbi Eliezer 36:14 combines both answers:
On that account had Rachel stolen them, so that they should not tell
Laban that Jacob had fled, and not only that, but also to remove
idolatrous worship from her father's house.
Sefer HaYashar Bereshit Vayetze 5 notes that it was, ultimately, pointless:
And Rachel stole those images of her father so that he should not be
able to ascertain whither Jacob had gone. And when Laban returned
home he asked for Jacob and his household and they could not be found,
and he went seeking his images to find out whither Jacob had gone.
And Laban went to other images and making inquiries they told him
that Jacob had fled to the house of his father in Canaan.
So, yes. This is a very good question, controversial amongst the commentaries and hard to see if we really do have a good answer. Still, there's a strong case to make that she had no obligation to destroy them, and stealing them was more effective in weaning her father off of them, which hopefully answers the main thrust of your question.
