Not with how high the DR on Defense is after 30% and the, what, 25% up time? Essentially taking the Defense Click relic, for an Assassin/ Shadow is giving you a second, really weak Deflection. Not worth it when it only amounts to an extra ~79 defense throughout the fight.
I simply have to facepalm at this statement, because it is completely ignorant of the actual usefulness of the click relics. Yes, they provide mean performance over the course of a fight. That's simple math: their rating contribution divided by their uptime is worse than the passive relics. *However*, that's not the entire story.
Unless a boss's damage is *entirely* flat the entire time, not varying appreciably based upon tank swap scenarios or different phases, calculating mean mitigation as if the click relic were passive provides an inaccurate account of its usefulness. For fights where you're not simply taking the same amount of mitigatable damage the entire time (re: pretty much every Ops boss there is), you're going to get more out of the click relic than simply the averaged calculation. Even factoring in "wasted" time while the CD is up by you're not using it, on fights where there is a large amount of off time for a tank (off time defined as "not getting attacked by M/R attacks"), you'll get more out of the click relic than the passive relic.
Why is this so? Here's some math to consider: let's look at the first boss of TfB. It's a pretty simple tank swap fight that has 2 tanks alternating between having aggro on the boss (getting attacked with M/R attacks) and waiting for an add to spawn (which takes like 10 seconds to kill with 90 seconds between each). Tanks have an uptime of roughly 55.6% each (90 seconds on boss, 10 seconds on add, 180 second cycle between tanks; 100/180 = ~55.6%). As such, you're only getting, normalized for uptime, 62.83 defense rating out of that passive relic (113 * .556). Now, let's assume you use your click relic every 135 seconds (average 15 second wait time between uses). That's 14.8% uptime for the Campaign/Dread Guard Imperiling Serenity relics or 22.2% for every other use relic out there (including the Rakata Imperiling Serenity relic, which is why I opt to use it instead of the Campaign variant). Since we're already factoring in downtime by dint of the CD, we just multiply this value by the given ratings (315 for Camp Imp, 290 for Rak Imp, and 380 for Camp Shroud) and we get 46.6, 64.4, and 84.4 ratings, respectively (I always kinda laugh when I see that the Rak Imp relic provides more than the "upgrade").
On fights where there is more downtime, the weaker the passive relics get and the stronger the click relics get. Unless you're tanking an M/R damage boss *constantly*, you're never going to get the full value that you think you're getting out of the passive relic compared to the use relics.
It's for this reason (different ratios of tanking uptime and different ratios of M/R to F/T attacks with different damage/risk weights), that it is simply stupid to say that there is a single best type of relic loadout to use. Every fight has a different optimal relic loadout. For some fights, it's going to be double passive relic (or a passive and a proc, depending on whether you like the proc or not). For others, comically enough, it could possibly be double click relic (using one only puts the other on CD for 60 seconds so you can use both without impeding the other if the on/off time is 30 seconds each). For Shadows, it gets even more convoluted because the healing relics provide some degree of functionality (the only times they're really useful are those fights with a large amount of F/T attacks and/or exceptionally low damage that brings mitigatable DtpS below a certain threshold).
Overall, unless you're bringing in and switching out every single relic you've got for each boss fight depending on the relevant mechanics, you're never going to always have the best loadout. As such, it's up to the tank to determine what they think the most important loadout is. Are they willing to sacrifice some mean mitigation for increased specialized mitigation (whether burst or F/T)? Do they prefer the passive to the proc? It's all about preference. As long as you're not bringing in DPS relics, you're going to get *something* out of each, and they're similar enough in average effect that it's not going to make a massive difference across an entire raid.
Also, when calculating DR and the relevant contributions, it's important to consider losses to your other mitigation stats. Yes, Defense has the smoothest DR curve, but the higher your Defense gets, the lower the value of all of your other mitigation gets. With a 30/65/60 spread with 40% DR (74.38% average mitigation), 2% Defense additional defense (~100 additional at ~500 rating) is going to have you end up with 75.11% average mitigation whereas 4% absorb (~100 additional at ~500 rating) is going to have you end up with 75.47% average mitigation. You end up with better mitigation with Absorb as opposed to Defense because, unlike Defense, Absorb doesn't degrade your other mitigation stats and, even though it gets hit harder by DR, it still provides more on a percentage gain per rating point scale. Until DR hits Absorb so hard that it provides only roughly 1/(current shield chance) percentage gain per rating point compared to Defense at its current level, Absorb will still be better than Defense.