After bug 21931, summary is always auto-filled when creating or replying to a thread. You should be able to add a customized summary to the auto-filled one, as with all auto-summaries.
Summaries are very useful in discussions to let people know what's happening without having to read everything (there's bug 20541 but I don't think that we'll dump recentchanges) and they're heavily used, see e.g. http://en.wikipedia.org/w/index.php?title=Special:RecentChanges&hideanons=1&namespace=1 (and if you're not convinced run a query on the database).
You can say that when you create a thread the title should be the summary, but this doesn't apply to replies.
Version: unspecified
Severity: enhancement