Blue doesnt often get uncounterable kill spells, and this is a way to just shut down Murktide Regent, Delver of Secrets, or Dragons Rage Channeler early and without fear of reprisal
Even if you're not playing a Kindred theme that will just take Radagast out of consideration entirely, you will eventually hit a happenstance of creatures in play that will have you putting creature cards on the bottom, not to mention the likelihood that you'll pull all non-creatures
I can equip, enchant, and otherwise target a hexproof creature like [c]Geist of Saint Traft[/c]
Slot in burn spells to churn through any resources your opponent's thought they had because the miracle of to pay for any one spell on any turn is powerful
Instead, Arden Angel is a flavorful throwback to old-school Magic
Apply rule 702.26 g, and Spirit Mantle phases out indirectly