Yeah, I love this idea too, but its limits definitely would have to be able to be tailored to fit each user's preferences.
With the ability to tweak this, however, it could prove to be very useful indeed for those who are interested. I almost never go over to another service to chat re; Steemit, just due to inconvenience and not total lack of interest. But, I would be somewhat open to that if it was integrated right into the platform here itself, within limits.
I'm not certain how content providers would feel about this though. It's work enough nurturing a fruitful discussion in the comments section that will actually garner upvotes these days; I can't imagine adding the implications of DM functionality to that as well.