Is Frozen Heart worth buying?

Given that there's a balance to maintain of effective health between your armor and health, since after a certain point you're better off just sitting on what armor you have and stacking more health, I've never once been in a position where I was playing a tank and thought to myself, "Sure I'd like 100 armor but no extra health to go with it." I know the 20% CDR and 400 mana are useful, as is the AS reducing aura, but as an item to help with survivability I'd much rather get something like Randuin's Omen which also has that nifty AoE slow active.

In what situations is it actually more beneficial to take just flat armor like that in a final item instead of mixing in health with it? A few of my tank champions could definitely use the mana, Rammus comes to mind and I'm trying to learn Sejuani but finding her quite a bit underwhelming in the new jungle, and I suppose Braum for when I'm playing support, but it feels like I'm giving up half of what it takes to be a tank for the sake of having some extra mana and CDR that I can get elsewhere.