sober - worthwhile discussions
If one has not followed a discussion from the beginning (e.g. at a mailing list), often it is necessary to read a lot of (scattered) text to understand the state of the play. Many people do not have the resources for this and thus
- abstain from taking part in the discussion or
- take part without knowing what has already been discussed.
Both consequences are unwanted if the discussion is intended to have consequence which the whole group can accept.