Besides the obvious examples of gathering a nice database to use for genocidal purposes... (sure lot's of idiots like to say that's overblown or not really a worry, while being alive in a world where there are on-going 'ethnic cleansing' campaigns).

There's also things like - the terms of service include the boilerplate "these terms are subject to change at any time", and I don't want those terms to suddenly change to "we will provide your PII to all insurance companies proactively in exchange for a kickback every time they are able to use it to reject a claim".

I already get hassled by the law somewhat frequently because my house used to be the residence of a criminal (2 owners ago it was used as a rental and that owner evicted said criminal). I don't want to add getting hassled by a bunch of people who came in below the max IQ requirements over someone I've never met because they're from "that side" of the family.