I'm not really sure how it's an exploit any more than removing Moroes's Garotte the same way is an exploit. Using an invuln-physical effect to remove an ability of School Physical is just game mechanics working as intended. It may be an oversight on Blizzard's part not to flag the ability as unaffected by invulns, but that's a different matter. If it was unintended, then the fight is just poorly tuned. My standard Kara group for the past few weeks has one hunter and one warlock and no other ranged DPS (it's not really some crazy skewed group composition, we just have two warriors and two shamans, and no mages, and otherwise an even spread), and a full load of 5-6 skeletons is simply not tenable when the bulk of your DPS has to melee. Every video I've seen of handling the full set of skeletons uses frost nova and primarily ranged DPS while healing is focused on a tank holding them in place. I may be wrong, and we simply weren't good enough, but I can't see how you do that fight without using BoP and with a melee-heavy group.
I think the fact that they're removing the Immolation at the same time that they fix BoP says as much. I'm definitely looking forward to checking the fight out in its 2.0.10 form.
On beta the group I cleared up to Nightbane with never figured out to use BOP to block out Bone Rain. When the guilds started clearing him out on Live quite easily, I thought, "Wow, they must be using tons of consumables!".
Truth was, BOP makes that much of a difference, I guess.
The best our group got to Nightbane was 50% unfortunately. Bone Rain + Immolate simply was too much. I knew of one guild on EU Hellfire that was able to kill Nightbane. I don't know if they also used BOP.
Back then, I thought the fight was clearly untuned. With Immolation gone, I think the fight has taken a good turn for the better. However, them being immune to all but non-holy seems a little overkill to me.
I think the issue came down to Blizzard would like you to be able to remove the damage from Rain of Bones but the problem is the damage and the skeleton spawning was together. Second, they realized not everyone has a balanced a raid mixture (in which case you would only have 1 or 2 melee beyond the MT which is no issue on the fight). Third, it was the fight that probably received the least amount of testing and as far as I know never really had any changes done to it, so its only now getting its retuning done. Where as fights like Curator went through many tunings.
tactic: until 76%, 51%, 26% only the Paladin heal (every healer heals the pala for mana. And NO HOTS, DOTS on other raidmember, nightbane). When Nightbane takes off, the pala critheal a warlock (lifetap) for 5-6k. The Paladin has aggro an heals the Raid with 2.0 Holy Lights + every Healer heals the Paladin.