does the -- button even get used? like, ever?

Frequently.

You use it to vote rants as “not for me”, “repost”, or “offensive/spam”

Ya boy

I've understood that you can train the algo with it.

I use for reposts

For repost, spam, sexism, racism, personal insults... Or else when I'm tired of seeing the same old college rants "I'm better that my professor" bla bla bla...
And yes, if I list these cases here it's that I've already seen them

For shit memes, wrong categories, reposts, anything incriminating/flammatory/rude. So yeah, I use it every single day

A lot in my case.

I have memes filtered out but still see them regularly so those are all downvoted.

Downvoting all those

- (shit)memes
- retards that want ++ by trashtalking languages / technologies that they don't fcking know
- the number of old jokes reposted 100 times a day fatter that my own mama