Larzi, put take the situation from a healer's perspective (with back of the envelope numbers)
40K incoming at 70% total mitigation leaves 12K to be healed
40K incoming at 75% total mitigation leaves 10K to be healed.
Take that over a few hits, nobody's dying yet. With 30K HP and 70% mitigation, you're at 18K HP and your healer is left healing 12K HP. With 25K HP and 75% mitigation, you're at 15K HP and your healer has to heal 10K. In both situations you're still safely alive, but with more mitigation, your healer has 2K (17%) less healing to worry about.
That's why people like mitigation over endurance, because tanks love their healers.
-Chapulin