severity 34071 important
The purpose of the BTS is to get bugs in Debian packages fixed quickly. Many bugs end up getting fixed by people other than the Debian package maintainers.
If interested developers could "subscribe" to bugs they care about, they would be reminded that a bug is outstanding whenever someone sent in an e-mail about that bug. This would encourage those interested developers to triage and fix those bugs. (I have seen this happen myself because of the popular and well-used CC feature of Mozilla's BTS.)
Once this issue is dealt with, bugs will be resolved faster because developers will get these e-mailed reminders. More bugs will be fixed per month. The release team will be able to release more often.
According to the debbugs documentation, the 'important' severity level is for bugs with "a major effect on the usability of a package". I believe the lack of a subscription feature qualifies: it makes the BTS do its job more slowly and significantly decreases the BTS's usability, despite the fact that this is a feature request. The lack of a subscription feature has a subtle but far-reaching effect on the quality of Debian itself. A subscription feature needs to be added to debbugs.