The problem is that some people just don’t like to be pestered. In some communities like the one I am talking about there will be a small group of active users where as others will only drop in when then they are working on a project.
For example lets say you have a community about camping. Most people will do their camping in the summer and will join in discussions about what equipment to buy or where to set-up camp. These people will only really be active when they are planning a camping trip. Others however are expert or hardcore campers who go camping all year round and want to talk about the subject all the time.
So some people are going to get annoyed receiving a digest with posts from the same few dedicated people every 2 weeks. However if it was possible to set the frequency to a longer period then people would be less likely to be annoyed by an infrequent email. But they will end up still getting an occasional email so when summer comes around again they will remember to visit the forum.
I just think that the limited choice doesn’t allow for situations that are not busy sites where you don’t want to keep pestering the infrequent users. Having the emails too frequent will just result in more people unsubscribing and then they are lost completely as they forget the site.