Why would this be true? IIRC, the defense talent does not increase your defense rating by 2%, it gives a flat 2% bonus to your defense chance. This would mean that it has no affect on the diminished return on your defense rating, therefore not decreasing the value of the defense stat.
This same holds true for crit chance % increase talents, which is why I do not like it when people advise DPS to "get your crit to 30% unbuffed". Crit and defense stacking should be based off of ratings, not percentages.
Please correct me if I am wrong.
This is absolutely true. However, a static increase in defense chance still affects the value of defense rating (even outside of DR) in the sense that the same defense rating boost results in a larger percentage improvement in damage mitigated. For example, imagine we have a defense chance of 20%. An addional 1% from rating gives us a 1.25% boost to mitigation (ignoring shield and armor for now). However, if we add the defense talent, our base moves up to 22% and an additional 1% from rating nets us a 1.28% boost.
Thus, the defense talent does increase the value of defense rating, albeit very slightly.