Sometimes, I want to mention someone within a card because I want them to see a particular comment. Doing so, adds them as a watcher and they then get notified about all subsequent changes to that task.
This frustrates some users. They could change their notification settings to solve this. However, even this for some is asking for too much. :S
It could be useful if there was an option to NOT create someone as a watcher for a task if they are mentioned in a comment for that task.