You've never met a call center.

They've sent bug reports with credit card data they've typed in during a phone call through a variety of insecure methods.

They've also written people's credit card info on sticky notes.

Trust me, the horror that is card data and a call center is scary.

How about a bug report screen shot? Lots of non-security conscious users don't understand why this could be bad. It's your (making the assumption that "you" in this case is a Slack Administrator) job to protect them from themselves.

The use case is user error. I have also seen no shortage accidentally people paste passwords into Slack as well