The last time I had to deal with this it was a result of links to a site from an email being attached to the user it was sent to (eg example.com/?email=foo@bar.com). It was not a wonderful example, just a quick one. The moral being by caching query strings you risk needing to parse the ones being passed to ensure there in not unnecessary caching.
I don't mean abusive in the sense that it's using technology in a way it was not intended to be used. I mean abusive as in actively harmful to the user's interests and privacy.