From a usability perspective, I understand why you're hesitant to add this, however for collaborative efforts, especially where a teamleader has assigned a task to two designers, this is NEEDED. Not fancied.

If usability is a concern, then have an administrative option to disable it.
It would only hypothetically be used where needed. It's not 'tagging' people in, it's making multiple people responsible for a task, which is currently happening, whether TeamBox supports it or not.