Tenacity affects both offense and defense. Because it affects defense, the devs will never purposefully let it be the most powerful offensive secondary. If you want to maximize offense and defense at the same time, go for it. If you're more focused on offense alone, other stats will serve you better.

If you full build ten towards the end of an expac you can get large, permanent damage reduction. Id need to recheck the math but it was like 10 or 15%.
It looks like full Tenacity in a maximum Item Level build results in 1920 (1940 if you're willing to get a partial return on one materia), which is a modifier of ~7.1%. Going for Tenacity with zero regard for Item Level (or putting Strength materia in accessories) resulted in 2924 on a Warrior; about 11.7%.