1) Does it add to survivability?
If your team can maintain their viability, without the tank dying or other players dying, during the 20 seconds of immunity... then in a broad sense, the debuff isn't necessary. The team has enough damage mitigation / healing to cope without the debuff. Indeed, teams would have to make sure of this before engaging the content.
So the debuff won't actually improve the survival of the team/raid. It's an 'icing on the cake thing' not needed in most situations.
There's one possible situation where it might be of more necessity, that I can think of. That is, immediately after a very large AoE nuke, you might hit the damage debuff to give healers a bit more time to recover. So a situational use at best.
2) Burst Damage
Most healers and tanks (and indeed players in general) prefer damage to be fairly consistent over time. This allows you to use your tools appropriately and get into a rhythm. Damage debuffs that for 10s reduce damage... then leave 20s with no debuff... then 10s with debuff... then 20s with none etc.. create a 'bursty' damage profile, which is often not desired.
I could see some raid leaders specifically asking that the debuffs in fact not be used in a lot of situations.
Nano-Cost
Nanocost on the top damage debuff is 3527. To maintain it for 20s out of 1 minute would cost you 7054 nano. To maintain it for 1 minute out of 3 minutes would cost you 21,162 nano. We know that we're not going to be able to maintain our top DPS profile as it is. Adding in the damage debuffs will only serve to reduce the available nuke DPS. When the damage debuff isn't going to directly affect survivability... most MPs who are trying to maximise their contribution, will use the nano for DPSing with nukes... rather than casting the bursty damage debuff.
Now compare this to maintaining the top unbreakable Doc Init/Dmg debuff, which is certainly comparable in effect. Maintaining that debuff for 1 minute costs 792 nano. But you can maintain it for the full 3 minutes for a cost of 2,376. This is much more supportable on top of Doc/Crat healing and nuking. Even if you have to cast it a few times to make it land through resistances... let's say 5 times per land, you end up with 11,880 cost. About half of the MP cost, for 3 times as much coverage.
3) Relative Contribution
In the majority of teams that are built for tougher content, where the MP debuff might be seen as useful, there are %age damage mitigators. That includes Reflects, Init debuffs, AAO debuffs, Evade debuffs and so on. Anything that reduces hit rates, essentially takes a %age of the damage away.
The extra contribution that is given by adding an MP to a team, is reduced sequentially by those %age effects.
Given a mob that hits for 5,000/hit and does 10 hits over 30 seconds = 50,000 damage:
Doc/Crat Init debuff -200 damage = 48,000 remaining
Init debuff reduces hit rate by say 30% of that 48,000 = 33,600 remaining
Evade buffs reduce hit rate by say 10% of tha 48,000 = 28,800 remaining
Reflect buffs reduce damage by 50% of that 28,800 = 14,400 remaining.
Total Damage Mitigation without MP = 35,600
Now add an MP with 2300 debuff and no immunity: mob now hits for 2,700 on 10 hits = 27,000. So it looks, at first, like the MP has saved you 23,000 damage
plus some extra reduction because of the extra init debuff.
Doc/Crat init debuff at least -200 damage = 25,000 remaining
MP Init portion reduces hit rate by 10% of that 25K = 22,500
Init debuff reduces hit rate by 30% of that 25K = 15,000 remaining
Evade buffs reduce hit rate y 10% of that 25K = 12,500 remaining
Reflect buffs reduce damage by 50% of that 12,500 = 6,250 remaining
Total Damage Mitigation with MP = 43,750
Difference gained by adding an MP = -10,150 damage
Total Doc/Crat init debuff contribution = -9,500 damage
So although the MP seems to be able to reduce each hit by 2300 and thus over the 10 hits would save you 23,000 damage. In reality, adding an MP to this team reduced damage by only 10,150 over that 30 seconds. If that difference were spread across the original 10 hits... that would be a 1015 debuff.
Now if you make that MP debuff only available 1/3rd of the time - then the MP contribution is about 3,383 over the 10 hits... 338 damage reduction per hit.
Difference gained by adding MP with 1/3 uptime debuffs = -3,383.... not 23,000 as it at first seemed
Total Doc/Crat Init debuff contribution = -9,500 damage... nearly 3 times as much.
Given that a tank will usually take the brunt of any attack in PvM and will always have either high reflects or very high evades... and there will usually be a doc/crat in the team for tougher content - providing their Init/Dmg debuffs oo... the actual contribution of an MP's damage debuffs to a PvM team is massively reduced.
Soloing Contribution
The MP damage debuffs presently find most of their usage in solo play, if we're honest. Perhaps also in small groups without the trinity roles well filled.
But at present in this situation, the MP can maintain his damage debuff of -920 per hit for 100% of the time. The 2300 value on the new ones can be maintained 1/3 of the time, so we'll see an average debuff of 766 per hit.... which is a good chunk less than we now have. There's more init debuff on the new ones, so maybe an extra 5% reduction on top - perhaps 820ish average damage reduction per hit on the new ones... which is still substantially less than we'd have with the old debuffs.
The burst issues of the new debuffs still apply in soloing - but this may be worked around to some extent and timed to fit in with cool-downs on our perk heals for example. So we can perhaps work around it better when soloing... but the basic issue of having less damage reduction overall, will essentially nerf damage debuffs for soloists.