Yes, that would be nice to have but probably not going to happen any time soon. I have logged this request in our project tracker so we at least don't forget about it though.
It is a difficult thing to implement because what a web app is good at is handling web requests, it is challenging and working against the grain to make it also be a task scheduler, and difficult to make it reliable. The right way to implement such a thing is as a windows service separate from the web app but for the vast majority of people who host mojoPortal in shared hosting use of a windows service is not feasible.
To make it work in a web app we have to do a more complex implementation to make it handle failures and restarts that will happen if the app pool happens to get recycled while its working and kills the task. So we would have to keep track in the database of each message sent to each user so that we could resume sending messages without sending duplicates. We also have to make sure we spawn one and only one thread to process the task in the background when the app restarts.
So, yes it would be nice but its a fairly big job to make it work well and there are too many priorities ahead of it so it isn't likely to get done in the near term.
Also I do think the real time notifications have the benefit of promoting interaction, a person's question may be answered sooner with real time or could be delayed if people don't see it until hours later in a digest email.
One current option to reduce in box noise though is to not subscribe by email but just subscribe to the RSS feed with a feed reader. Though google reader went away I'm liking the new aol reader and there is also the new Digg reader.
And of course there are techniques to make all the messages go to a specific folder in your mail client using rules about the from address or what not, so that is another option to reduce inbox clutter while still subscribing