The defense-buff for dps is quite useless in most cases. If a mob has 2 million attack and you increase your defense from 500.000 to ~ 540.000 this reduces incoming damage by around 1.15%.
The defense boost is valuable for high defense members like Tanks. A tank going from 5mill def to ~5.310.600 reduces incoming damage by around 6.2%. However, if your defense value increases from a value 5 times higher than mobs attack, it becomes useless.
For example if mob has 2 million attack and you boost your defense from 10mill to even 50 mill, you get 0 benefits. If you go from 9.5mill to 10.5mill, you only benefit for the part going from 9.5mill to 10mill.
The same logic allows us to understand why Warden Pets are strong in high def inis, but very weak in other.
Obviously, you can still take defense buff as dps for 6million for the slight damage-reduction it provides.