Blizzard's stance on the mount is the same as for legendary weapons: to allow it to drop in 10-mans would detract from the item's uniqueness. Presumably, they don't want 25-man raiders farming 10-mans for additional legendaries and, thus, the mount: it would increase each 25-man raider's chance substantially to see the mount because they can hit up both the 10 and the 25 version.
Yet, we have achievements that are capable of sensing whether a raider in the group has been to 25-mans. Why not limit the 10-man chance to drop to groups who have cleared the instance without any member of the raid ever having stepped foot in the 25-man? There are multiple ways of doing this, including the gear-check ala Dedicated Insanity or just a "this person has killed Marrowgar 25."
Certainly, once Cataclysm is released, 10-man guilds can go back into ICC25 and try their luck at getting the mount. Just as they can all go back into TK from TBC and try to get the Ashes of Al'ar. *snort* How often does that happen? Most of the time spent in a raid instance is during its height as content, and even now, TK is beyond most players due to its complex mechanics, many of which are built to assume a full 25-man raid, and the drop chance is still infinitesimally small (edit: that is Al'ar, of course; Mimi-head and Invincible are 100% droprate assuming you can make it through the entire heroic-mode dungeon/hardmode). Is it fair to assume ten-man raiders will have to pug a 25-heroic version of a dungeon to get the mount, or have to do it when they're in Cataclysm? Will the mounts even still be available then, or will they be yanked out of the game like the Amani Warbears?
I don't care so much about the legendary weapons. Those are equippable items that impact the raid's actual performance, and to allow them to drop in a ten-man (even with a "strict" drop criteria) would require the coding for a "lower item level" version of the current item, which in turn seems counter-productive to the weapon's existence as a legendary. In the end, the weapons get replaced, shoved off into the dusty corners of banks and taken out to play fashion-craft with outside the auction house.
Invincible is different. This is a mount. In Cataclysm, while the healing legendaries of Ulduar are rotting under a mound of virtual cobwebs, Invincible will still be a glory symbol carrying players across the skies. Mounts are useful. We still see the Al'ar mount and sigh in jealousy, or the Headless Horseman's mount, or Rivendare's or Attuman's or the black AQ bug. We even still see the RARE original mounts of the game owned by players who have been in since day one, and didn't turn them in for the graphic upgrades. Mounts live on in a way that legendary weapons don't.
Mimiron's Head was handled the same way as they plan for Invincible: it only drops off of the hardest mode of 25-man. Ten-man gets a trinket that summons a tentacle, which is admittedly pretty cool. There was less of an outcry against this because Mimiron's Head was, as a mount, kinda corny. It doesn't have the same awe factor as the Ashes of Al'ar or Invincible. It still wasn't very fair, but it wasn't in that same way of "wow, I want that... for me, or for my guild."
Blizzard has stated many times that they support and believe in ten-strict guilds as being viable and non-casual, but at the same time, they limit us to the subpar cool-stuff. Legendary weapons we can live without, but when it comes to really awesome mounts, why are we getting shafted, especially when we know the code exists to limit its drops to only those ten mans who ARE strict?
Patch 7.2 for balance and resto druids
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