This is actually not great, because everyone else doesn't benefit, because it's in a DM that nobody else can see. In an office people near by will benefit too by being in earshot.

Don't know about you, but I generally don't speak loud enough that people outside the conversation generally overhear things. Also, word-of-mouth is a absolute horrible strategy to make sure things are shared across a team.

But you can copy all of the contents to tickets, emails, etc. All is not lost.

I've done the same in chat rooms where everyone on the team can read if they want to, even if the conversation is limited to me and another individual.

Yeah I think there's a place for both honestly. The place I work at encourages using a channel. But when it's deep and freeform, I think DMs is better. Sartre's Look and all that.