Blizzard Removing Rusted, Ironbound Proto-Drakes
According to a post by Bornakk on the official forums, the rusted and ironbound proto-drake rewards for the Glory of the Ulduar Raider meta achievements will be removed in the future, possibly with the release of patch 3.2. Whenever the team decides to remove the mounts, they will provide about one month of warning so players can try to complete the achievements one last time.
So why are the proto-drake rewards disappearing? Well, Bornakk said the mounts are meant to be rewards for the top tier of raiders, and the introduction of new and more powerful equipment into the game will make the completion of the meta achievements "easier than intended."
Bornakk also said the team plans to continue removing similar rewards in the future. So basically, once a high-end reward becomes too easy to get, it can no longer be obtained. You can read the full announcement below.
The rusted and ironbound proto-drake rewards for the normal and heroic Glory of the Ulduar Raider meta achievements will be removed at a future date. We currently do not have an exact date they will be taken out, but it is possible they will be removed when patch 3.2 comes out or at some point after that. When we do decide to remove these mounts we will provide about 1 month of warning so players will have time to make some final attempts.
These mounts are designed similar to the Arena Gladiator mounts as rewards for the top tier of raiders and we will remove the rewards once we have introduced enough new and more powerful equipment to make the completion of these achievements easier than originally intended. Since these rewards are designed for such high-end game play, we currently plan to continue removing any similar rewards we make available in the future.