Member of the chat team here! 
As @Falco shared, we’re actively working on improving and expanding chat so that it can support the unstructured, off-topic conversations you mentioned.
We recently completed a research project with some chat users on Meta where we learned loads about what people love about Discourse chat today, and how we could make it better for them going forward. A lot of that research is informing what we’re working on today.
Here are a few examples of projects we’re working on (disclaimer here that this is what we are working on today, but in the world of software things change fast so no promises about timeline at the moment):
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Updates to group chats (direct messages with more than one other person) so that you can easily add new members to an existing group chat and set a title to help distinguish that group chat (or just have some fun).
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Improvements to chat’s mobile design so chat is easier to browse and interact with from your phone.
There’s more to come! Further on the horizon, we’ll be tackling projects like improving notifications so that folks can stay more well-informed about conversations they care about.
I view “DMs” (or “PMs” as we tend to call them) as a substitute for email. It’s slower, more formal, and longer lived than chat, which makes a good option when you’re having an asynchronous, more formal conversation that you’d like to have a long-term record of.
Chat, on the other hand, is — as you’ve mentioned —
I’d also add that it’s better suited for synchronous communication. I could see someone choosing a 1:1 chat over a 1:1 PM when they want to work through something quickly with a colleague or want to catch up socially in an informal way — we do both of these a lot at Discourse!
While you could use just one or the other, they offer specific advantages that make them better suited for particular use cases — so having both at your fingertips gives you more flexibility in how you communicate.