I still consider it on high damage AT's as long as I don't have to sacrifice any or much actual damage enhancement. You'll hit ED on damage easy pretty much, so the only thing you save by not making the 6th slot a damage proc is something that matters less if you are all IO'd out (little extra accuracy, end reduction, or recharge). This is especially true with purples, which WAY overenhances damage generally. You are better served by skipping the plain damage IO in those sets, eliminating the 6th slot, or putting in some sort of proc, whether damage or otherwise, and just boosting to +5 the Dam/End or something to get to ED (even without boosting, you are still enhancing it to like, 89% or something). TL;DR, it's just a way to break past the ED damage limit. Or have some other utility like -res or FF+recharge etc. Obviously this pays off way less on high recharge attacks, so usually I 5 slot those (or just use the ATO there, since the set bonuses are good, but they don't necessarily have a proc).