Hi Ryan,
It is up to each site about whether or not they want to enable digest email to begin with, and many communities prefer to keep digests. They are a good way to keep tabs on communities that you check in on every once-in-a-while.
You aren’t going to get these forever if you ignore them. After a configurable amount of time (default 1 year) if the site has not seen or heard from you, it will stop sending digests. Maybe like @pfaffman suggested, we could consider turning the defaults down slightly there.
There is no way of knowing how each user intends to interact with a given community. We make a good attempt to set good defaults across a wide range of users, but there’s no way to ship a one size fits all.