For T&C, it actualy mitigates quite a bit of damage while off-tanking (Or, not actively tanking anything, such as for tank swaps. Off tanking refers to any tank thats not actively tanking harder hitting mobs/bosses.) For paladins, its essentially 2 extra Sacred Shield absorbs per 6 seconds. For warriors, its an extra Shield Barrier per 6 seconds. DKs, extra rune tap per 6 seconds, and monks, an extra Expel Harm per 6 seconds in terms of healing done. It absorbs an amount equal to about 2/3rds of the druids current AP.
T&C only mitigates autoattacks and only from the target it's applied to. It can stack like the DK's Blood Shield, but if you're not actively tanking, your AP will be lower so it will still not be as high as when you've got the boss on you. It can be a life-saver when it's stacked up during one of Ji-Kun's channels prior to Talon Rake.
One thing I'd like to remind you, when we're talking about Guardians tanking Ra-den. Since both Savage Defense, Frenzied Regen and Tooth & Claw has a steep rage cost, it's not like we can just spam all 3 of them whenever we see fit. You can't just stand there and spam Frenzied to top yourself off non stop, cause then you won't have the rage to use Savage Defense before a Fatal Strike. Toot & Claw is a proc that is tied to Maul and Maul costs a ton of rage. So, you won't really be using T & C much outside of dps CD's like Berserk or Incarnation.
Also, Savage Defense is on a 3 charge system, meaning you can't have it up during the entire encounter.
The problem Guardians have with "tank -killer mechanics" is that Savage Defense isn't 100% guaranteed to dodge. Tortos Snapping Bite is a good example. We can have our Savage Defense active but if we don't dodge, we still take the full blow to the face.