Druids, due to the nature of thier spells, generate an EXTREME amount of overhealing on most fights. This can be easily seen by taking a look at Spine Heroic logs, Tsulong logs, ect. If your HoTs can heal for full, with no overhealing, Resto Druids easily outpreform all other healers by 50% or more. Of course, for most fights, all this healing tends to go to overhealing, and fights that have unpredictable burst damage are fights that resto druids aren't suited to.
This change will allow us to fix 2 of our problems- our inability to deal with massive incoming damage waves (which doesn't happen a lot this tier), and our overhealing from our HoT effects.