Let's make some numbers appear. You're in a high level instance, fighting a boss. You and your Co-Tank are running mitigation builds. Each of your DPS are putting out approximately 500 DPS. You and your Co-Tank are each putting out 200. Since you're beefy and survivable, both of your healers have many chances to throw DPS spells out. Let's say that they're each putting out 40 DPS, which takes into account misses.
This is a total of (500 x 4) + (200 x 2) + (40 x 2) = 2480 Party DPS.
Now, let's say that both you and your Co-Tank decide that you're surviving more than well enough in the fight, so you're going to go all in on STR. You both manage to increase your DPS by 10%. You're now both tossing out 220. However, your healers now need to work harder to keep you both alive. They can't DPS as much. They're now contributing 30 DPS to the fight each. The reality is probably lower, but let's highball them anyway.
This is a total of (500 x 4) + (220 x 2) + (30 x 2) = 2500 Party DPS.
Your efforts at stacking STR have increased your group's DPS by 0.8%. To this, I ask: How much faster will this really end the encounter? You'll end the fight 0.8% faster. You'll end trash fights that go for an average of 1 minute less than a second faster. But, bosses are what's important here. Let's take a 10 minute fight. You've ended the fight 4.8 seconds faster. However, fights these days are even longer than that. Let's go 13. It ends 6 seconds faster. So, yes. It adds up eventually. However, what you'll end up mitigating by missing those very few seconds is quite random. It could just be an autoattack. It could be the Tank Buster(tm). Or, it could be the actual Enrage mechanic of the fight. Some fights are indeed heavily scripted, but human performance is variable enough that you'll never finish a fight at an exact minute and second every time. So, that means that contributing 0.8% more damage to the group at the expense of more than 0.8% of your survivability for the purpose of mitigation has an element of uncertainty. Sort of like... Parry! You never know when a Parry is going to happen and you never know exactly what the boss would have done to you if it were alive for another 6 second. However, Parry can happen over the entire course of most fights but this only happens at the end. If your Parries in a fight would prevent an amount of damage equal to the amount of damage the boss would deal to you in 6 seconds, then it's smarter to use Parry than Strength. Regardless, defending this method of mitigation is, as I've said in a previous thread, hypocritical of the argument that Parry is not worth using.
But, no matter what, and this needs to be made absolutely clear, the benefit of the Tanks dealing 10% more damage each in the full party situations where you're deeming it most useful is so, so small in comparison to the survivability they lose that there is literally no reason to sacrifice so much survivability.