Firstly, people know about deafen, and tend to react accordingly.
Secondly, this works against communes (almost certainly, since the skills with similar effects do too).
Thirdly, what makes this nice is precisely because it isn't guaranteed. If you need to deafen, serpent strike with a whip. If you want someone to stick around and fail a word of recall, use this. In particular, this comes with a nicer amount of damage, and will often catch someone unawares because they were not taking any damage whilst you prepared to make the timed attack.
