It's already pretty powerful, it practically stops any ninja in their tracks for even a breif period, so in any combat situation where one/multiple is snagged for even a few seconds, that few seconds will result in a quick series of easy kills for comrades.
Besides which, we only seen it resisted by ninja with gigantic amounts of Charka (The cursed seal makes the user have much more charka then they would typically have) and S ranked ninja (Hidan, and even he couldn't break out of it the first two times it was used on him without assistance.). Every other ninja that has been hit by it has had to wait for him to run out of Charka first (even when worn down from his pervious battle, he held around 7 ninja for a decent amount of time.)
He's simply been using it on ninja that are above his league, the fact he was able to completely stop them for doing anything for minutes at a time is more then enough time for supportive elements to move in and kill whoever is being held. If anything, short of bloodline techiques, it's one of the more powerful abilities in the manga, if orientated towards a different role then most destuctive Jutsu.