Can someone explain defensive disc?

[Troxx]

Quick note: Evasive is by far the superior disc unless you’re main tanking a heavy hitting raid mob. You could make the case that defensive could be safer if trying to one group a heavy hitting mob with few assisting players or inadequate healing.

But yeah evasive is the way to go for group encounters. 30% faster cooldown and (most of the time) superior damage reduction.