Well most grimm are pretty mindless, so I doubt they'll bother avoiding a cloud of darkness/disease until it's too late. Think of it as Crowd-Control; it keeps your enemies were you want them, which depending on your team comp, could be away from the rest of your mates.

[@Mr Allen J]

If here still around, I still got my lil brick right [url=http://www.roleplayerguild.com/posts/3961334]here[/url] for yer purview.