The point is to maximize party DPS, not necessarily your own DPS. The problem is simply when broader tuning issues are so out of whack that even forgoing your ~4000 healer potency (~5000 in tank potency) per minute in shielding per TBN oGCD... still might not make enough of a difference to actually force out a GCD heal on you or someone else later (due to using on you a bankable oGCD more than they scheduled out), meaning that the defensive may produce nothing.
If the TBNs were to each, say, produce at least 1 extra Dosis cast made possible, each of which does 429 tank potency in damage atop 221 tank potency in healing (since healers' output is all multiplied by 1.3x relative to tanks due to their Maim and Mend traits and tanks' lack of anything similar), in addition to saving healer MP, TBN would be worth casting any time it could be maxed out even if it didn't grant Edge/Flood on break at all.
As for why people don't worry as much (or rather, since TE was nerfed from its media tour pre-release state to its state since) about SAM needing raid damage every 15s in order to deal its max DPS...SAM has considerably more flexible weave space and the Third Eye's mitigation is frankly pitiful so it wouldn't need to forgo taking advantage of a tickle per 15s to mitigate something near-fatal that'd come up slightly outside of that rhythm. It's just not big enough to make a difference given that SAM already has more HP than certain raidmates who don't have any personal defensives, period (like Rangers, DRG, or RDM).
As such, Third Eye has no conflict between defensive or offensive use that would cause one aspect to be overriden by the other... because it's already purely an offensive tool. It's already that thoughtless. It has no decision-making or impact outside of its up to 40 gauge (worth 452 effective potency) per minute, only some minutia in learning how early you can get away with pre-popping TE in order to grab another 10 gauge as little as 12s later (though still for that max of 40 gauge per minute).
All that being said, Third Eye is arguably an example of how a defensive ought not to be designed (by making it no longer a defensive or leaving it so influenced by wider imbalances that its defensive value is redundant/irrelevant). It doesn't excuse that design; it shows it'd be wasteful even on a Melee DPS. Putting that on tank is not a great idea.