I personally view getting active mitigation right as critical. However, my guild recently was having issues getting enough raid dps and especially enough ranged dps. Since we had numerous people with tanks, I volunteered to let them tank and I would see if I could get a ranged dps toon ready. We did get three new fights down once I made that change, but of course, I was really looking at WoL to see what all really changed. The bear that started tanking for us was keeping Savage Defense up about 8% of the time...yes, 8%. Yet, we still downed three new fights.
Based on that, my conclusion has to be that good active mitigation tanking is just the difference between good and great tanking. Good healing, sufficient dps and certain mechanics checks are more critical to downing fights.
Having said that, I still feel that good active mitigation tanking is important to me and I will still strive to do my job the best I can. I take pride when healers tell me, "I barely have to heal you".
"he doens't need healing, he doesn't need healing, he doesn't nee-WHAOSHIT!wtf was that man!". Please stop leaning on TDR. -Teng