Ok after chatting with others in the Diadem I think we've firmed out the primary problem. While being short a DPS or two is one issue (which may or may not need "fixing"). There is a serious issue when adds spawn around a large mob encounter. The problem is if any group attempts to DPS adds down they will loose kill credit on the large mob!!!

So it becomes not just optimal but necessary to let adds run around rampant. Often times the groups which get focused by said adds can loose their kill credit due to DPS deaths by adds.

This appears to be the primary problem for why kill credit is lost. Likewise its terrible because it does NOT promote healthy interaction between the groups, teamwork for properly controlling and killing adds, or keeping adds off healers.

This makes being a healer in the Diadem rather sucky right now since you're basically expected to face tank adds and heal the group.