Seraphim is an interesting choice, and if I only wanted to pad my personal meters I would probably try it more. But what is the tank's job really?
No, I don't mean tanks shouldn't worry about dps; I am not of that school. What I mean goes deeper than that. Tank's job is to control the fight, to facilitate and assist top performance by rest of his team. Take any size pull I can survive with Seraphim, and I can go bigger with Last Defender. Last Defender scales with size of pull. I already burst over 1 million with Last Defender on trash pulls, and compete with my DPS for total damage at the end of the instance. But we should be judged by how well we set our teams up for success, not our personal numbers. I can hold more mobs with Last Defender AND because LD Is entirely passive, I can focus more on other factors - having SotR up for big hits, interrupting and stunning, etc.
All that said, I am humble enough to realize there are other prot pallies better than I am. LD works for me right now, but I hear ya that Seraphim is working for you. For all I know you're just better than I am. And who knows? Maybe I'll even change my mind before this is all over.
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I use Final Stand for necrotic. Having one more way to drop stacks becomes more valuable than the extra heals form HotP. Especially with Teeming this week, those stacks will add up fast.
Blessing of Spellwarding, Final Stand, Consecrated Ground, and I like Holy Shield for Necrotic as well. Not how I spec most weeks, but probably how I'll roll this week in M+.