I think the idea is good for those threads that:
1. You don't really keep track, and
2. You don't want to be notified of all replies (which is what I think thread subscription does, correct me if I'm wrong).
Sure, you don't get notified if the quote tag is not fully used, but it's still better than not having any notification at all if you don't want to subscribe to a thread or keep track of a thread manually.
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