Personally, I find reply via email most useful when I am very crunched for time, but a critical individual post appears that needs a relatively quick answer from me. It’s faster to fire off an immediate email reply to the email notification than it is to click or tap and load up the browser and the full site.
The one-off nature of the message (versus having a bunch of topics I need to reply to in a group, where it would make more sense to load the entire site and compose several replies in a row), and the “I’m in a remote place with limited access”, are both essential ingredients.
In summary I’d say not very often, but when you need it, you really need it. It’s in the nice to have category for most people.
(The weekly email summaries are vastly more important for general engagement. Discourse automatically emails a “greatest hits” of what’s going on in the community and invites people to become intrigued, click through and visit to get a fuller sense of what’s happening. These go out when you haven’t been on a site for 7 full days, and they only happen for 26 weeks in a row before concluding the user probably isn’t coming back, especially if they haven’t been seen on the site a single time in six whole months..)