Warmog's in general didn't have a great place in Dominion. Understand first that, as a whole, defensive stats in LoL tend to be under-priced compared to offensive stats. This posed a good bit of a problem for us in development, but ultimately we were able to mitigate it by adding the map-wide armor and magic penetration buff. Warmog's as a pure HP + HP/5 item is a defensive item that wasn't affected by that change though, so it still had that beneficial pricing. Further its core weakness, needing to build it up over time, it heavily mitigated in Dominion due to the number of assists that are generated, and in general it was the only real remaining "farm up" item we had on the map.
We had been planning on replacing Warmog's with a new defensive item for awhile now, but with the community starting to pick up on how good an item it was on the map we opted to pull the trigger now even though the new item is not ready. While I do wish we could have shipped the new item as well, we generally thought the game would be better off at the moment without Warmog's.