Less hardcore guilds that are only now starting to clear ulduar will not be getting 5shards pr week on average (which is what it'd take to get two hammers). Add to that the fact that less hardcore guilds usually have bigger problems with turnover/guild splits and that they will be doing significantly fewer hardmodes and less chance of dual-clears (10&25) (I do think the drop% is higher for hardmodes) you are way off course here.
Other then that I agree with your post, but this hammer will not be seeing any large quantities in any guilds that are not bleeding edge (think 4hm t3&tf-warrior transfers in classic), any time soon.
Sorry I didn't make myself very clear; I was thinking more about guilds that are clearing stuff now rather than in the first 2 or 3 days. My feeling is that guilds that brute force encounters will still brute force them without this but those that stick to their normal schedule and will take more than a month to clear all the hard modes will find these extremely useful for progression.
Actually in reality it's no-where near 23 seconds. Apart from Sapphiron where is the AoE damage to the raid constant? My limited experience is that I won't heal any one person more than about once every 10 seconds even when things are crazy. Also the individual shields will be weak. As mentioned above a 6k heal, which is big for AoE healing gives a 900 point shield. I have yet to see an encounter where that makes the difference, what makes the difference is people not coordinating healing or someone standing where they shouldn't. To get meaningful raid protection out of this proc, whenever it happens to be up you will need to spam very high cost spells.CoH/wildgrowth aren't going to cut it in the making a major difference category, either you have enough healing without the mace or your wiping anyway. An extra, maybe useless 500pt shield to random dude every 45 seconds isn't worth the price of the proc. It's very similar to the value of a discs priest Aegis, which is in raid healing terms a complete non-event.
Certainly my experience raiding on my priest has been somewhat different. At least being one of 5 healers on Kologarn/Hodir I was finding that I needed to and could constantly spam PoH and the fights were over fast enough that even with constant casting (Kologarn was extremely laggy for us; yay for instance servers) mana wasn't an issue.
Also with paticulary healing intensive fights normally one healer is on the same group of people anyway (ie: melee/ranged camps even group specific sometimes - mimiron phase 2) so this mace would shine there.
There is certainly nowhere in naxx you'll get that much usage out of it without running at 3 or so healers though.
From what I have seen, hardmodes are only about pushing near impossible DPS numbers from your raid and outlandish HPS on your tank. It's got nothing to do with raid damage. Hard-mode wise, the mace proc will be very valuable on your MT, because it should allow you to drop a healer in favor of another DPS. Not because you can shield your raid from a few thousand damage.
I had Hard Mode; AoI: Runemaster, Freya, Mimiron, Hodir and Thorim in my mind as having alot of raid damage as well as the rediculous DPS requirements but this is just obsevational and theoretical from my stance maybe I over-estimated. I guess someone who has experienced more of them should clarify but I would of still thought it would be more useful than a shield on the MT? The relative % wise mitigation would be more surely as many tanks are getting to in excess of twice many DPSers HP.