Just re-read chapter 510 where Tobi fights Konan and uses Izanagi. He says "A techneque that can only be used by someone possessing both the powers of the Uchiha and the Senju".
But later when Itachi fights Kabuto and uses Izanami, he explain that Izanami was a jutsu created to counter the use of Izanagi. Since it was spammed so much by the Uchiha. But how the hell could they "spam" Izanagi? Tobi says you need both Senju and Uchiha powers, but yet Uchiha still used it with only the sharingan. And one person could only use it twice, and be blind as a result. So how could it possibly have been abused? Didn't they think that loosing even one eye would be a bad thing?