You're correct that passive defense doesn't do anything for the individual dps player. And in groups formed entirely of dps players, requiring them to use some passive stats won't change how they play at all. But that's not the point. The point is by giving room for groups of diversified players in which some can then focus more on dps by using glass cannon specs and gear, and allowing others to use active abilities to make up for their teammates' lack of passive stats. In this model you can choose to have a bunch of dps players who all use balanced stats, or for those who want to have diversified roles you can choose to do so and have it be at least as viable. (I say at least because I think that ultimately people who have diversified groups should be rewarded by being slightly more effective, because creating those kinds of group synergies is more difficult. But I would settle for just as effective.)
Some of us would prefer to not be in groups where everyone is just dps with different utilities. I understand that you're not one of us, but I hope you can understand our arguments. It's about allowing different playstyles to be used in different levels of skilled play, as opposed to just being relegated to the unskilled or in situations where people haven't learned encounters yet.