I never bothered looking if Runic Corruption was additive or not, since I wouldn't use a Death Coil after getting the proc. I did bother checking it out, and you're absolutely right. Still, even with this being additive, you probably don't want more than one at a time, since a single one is enough to get you enough runes to use due to the current GCD and the average Haste people want to stack, but it's still a good counter.
Regarding the macro, you'd still run into the problem of using up RP while having runes up, which would end up being an RP waste in the end if you end up rune-less and with no RP due to always spamming it. All it takes is a little bit of RNG, which during rather large fights, would happen once or twice. I still believe that better players wouldn't simply macro it to their SS, but they would definitely not let it overcap anymore to maximize damage, which is the intended change here.