I think the answer is “It depends on how your organisation is using email”.
One of the advantages of a platform like Discourse is that it provides what you might call “discretionary pull”, meaning that individual users can choose to pay attention to the full range of topics and categories on Discourse, with as much or as little notification as they see fit. Email is, in comparison, an explicit push technology.
I think of Discourse as sort of halfway between Email and Chat, but how useful it will turn out to be will depend on the discipline of your users, and what other central information storage mechanism you use.