Credit to Kyan for this:
Once the enemy has around 60 armor or more, LDR does equal or more damage than most ADC items. However, ADCs often need basic AD, crit, and AS items to start scaling and getting a good amount of attacks off in teamfights. LDR is almost always going to be the optimal third item on most ADCs. Sometimes it might even be the best second item, if you have enough attack speed already.