Board Thread:General Discussion/@comment-11276487-20130706160459/@comment-15145664-20130811203527

The same reason you would exchange Ebonyflesh for Dragonhide, because it's better.

No magicka cost, lasts several minutes, no damage intake, chance to rebound magic...

I could go on, but I feel that I've made my point. Besides, this shout would be good for players that aren't mages and can't cast the better mage armor spells or wards.