Dominion There's an issue for this. This feature will probably be added as a future upgrade of the Subscriptions extension.
That's good news. I've had a couple beta testers asking about this feature. It's a good feature for exactly the reason Arjun mentioned, getting users back that may have got caught up in their busy schedules elsewhere. But one clarification on that point, it should be an opt-in thing, so members can remind themselves as needed. The truth is, few members of any forum community will actually follow it closely via direct visits every day. There are those top 10 poster types, of course, but a digests offer in combination with thread follows (subscriptions) is very useful for interested people that are too busy (or can't remember) to check the forums every day.
isti37 it should be part of a marketing automation extension
Just the idea of a "marketing automation solution" makes me nauseous. This is community discussion software. We (CSF), as example, will forbid any marketing in our community. If people need marketing solutions, use G+ or Facebook. That's what they're for and will give you bigger reach. (But whether they do or not isn't the point, nor something I really care about.)
And a digest is not spam if a member opts-in for it in their preferences, as it should be. By doing so, they independently decided they would like to be notified by digest because it probably works for them better in staying informed of new discussions; discussions they might otherwise not see in time to give relevant feedback, or might never see at all.
