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.
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.