Tank Mitigation vs Tank DPS.

[Taika]

I will take these 2.5% because it will make me a better MT and i like to MT, and i think tenacity is really good for dungeons and MT tanks in raids.
Less burden on healers, could translate into better group performance.

It won't change anything about how the healers are healing in any content.

You could make the same excuse on D hit, since it is just a chance to get bonus damage, it could proc less often than usually and that 1% difference could be gone.

These are different cases because the DPS increase is an average and % based, while healing is based on the abilities your healers use and the amount of GCDs/OCDs they are using on healing. You melding tenacity will not change your healer's ability use in any way, so it doesn't affect the group performance. Because of how the content is designed (scripted damage, rare tank buster bursts, minor auto-attack damage, extremely strong damage mitigation and healing abilities and a lot of passive healing, and of course all the significant encounters being designed around an enrage timer), your higher DPS, no matter how small, will always be more significant to the group performance than a tiny bit of extra mitigation.