Its more useful than you think but still not great.

E.G with 2.4k Tenacity instead of 2.4k Det. You would have 6.1% damage and mitigation instead of 8.1% damage, so for roughly 2.1% less damage done you take up 6.1% less damage.

So not great to focus on, but don't be too mad to have some on your gear its not that awful.