phi algo is so tight on nvidias, there is no needs to do an phi amd miner. Like neo miner. It exist, but not many people use this
So far all I have to go on is whattomine estimates. Does two rigs of 1070s produce more than same number of 580s? According to 1612 #s yes, comes out to about 165 vs 200.
So if you're in it for efficiency then sure, AMD not a viable option. But realize some are playing the gamble in hopes it increases in the future. If someone can still cover costs, even make a miniscule profit, but achieve the goal of banking some coin cheaper than buying...then it's worth it to them based on their strategy. Looking at the future potential of what is mined, not the current going rate.
That said--i can't buy the "there's no need" line. There's hash to give and Dev fee to be had if someone produces something, that's the bottom line.