I disagree that it's a useful mechanic. It's a bandaid. It's a reaction to "hmm, druid HoTs get sniped and tend to overheal a lot. Mushrooms are boring. Oh, wait, let's have overhealing charge mushrooms!" It's not a terrible idea, but it's not brilliant design either and it doesn't get at the root of the issue - HoTs overheal because they're sniped a lot. And again, I dislike the idea that this reinforces that our AoEs are all about being in a location. It's fine if the fights lend themselves to that but if they're mobile then the druid needs to anticipate where people will be when the CD/AOE is needed, etc.