That math is very incorrect. You add the 50% paragon to the 20% on the amulet and then compare the 20% to 70%? Is that fair? Shouldn't you compare the actual trade-off, 20% elemental to 20% to 20% area damage?

Using your numbers 50% paragon area damage would be a 200k and 100k area damage hit. With the 20% AD amulet it would now be 140k. That's 40k more (same as the elemental...because 20% more is the same as 20% more obviously). You also only take it time 8 mobs because the person you hit does not take the damage, only the other 8 and that's why it's 100% worthless in single target cases). That 40k hits 8 people for 320k extra damage. But it only triggers 20% of the time, which is 64k extra.

More, but not as high as you were getting when you added in the paragon and the extra target for comparison. So in theory it's better, and if you do the math the cutoff for where AD is better than flat damage is actually 6 using this formula, but it's got a big lie in in because of the broken application. Look at it again this way.

I hit for 100k with everything but that amulet prefix. If it's 20% elem I hit for 120k, easy enough. The AD is tougher though. I would only hit for 67k without my Focus buff, the Focus buff is solely responsible for a 50% damage increase that multiples the damage from 67k to 100k. The area area damage is calculated as if I didn't have the Focus buff. So, I get 20% of 67k times (9-1) targets times 20% proc rate, or 21,333 more damage. Basically the same as the the elem prefix. At 8 people in range it drops to 18,667, or worse than elem damage.

That's why it's so bad that bonuses aren't being applied, it severely gimps the performance. It works, just not nearly as good as it looked when it was 100% effective. And, against single target people it is completely dead and I spend fully 10% of my time in GRs fighting Rift Guardians which almost exclusively single target. So, just go ahead and cut 10% off the effectiveness right there over a rift. If you find you're dragging 12 enemies half the time and 6 enemies the other half it might be about on par, but most people prefer the consistency. CDR is a DPS gain and usually a toughness gain. Attack Speed is the same. Crit damage is pretty consistent actually. These are all just more reliable stats.

If you want to fish for perfect rifts to push progression to the max there might a place for it because when it works well it works really well, but until the remove the bug that makes it just ignore buffs it's really tough to say it better than any other premium stat.