The point is you need a sa'angreal for this weave. That means either Callandor or the Choedan Kal.

Callandor was out because it has the flaw that makes using it dangerous. Did Rand know that at the time? I don't know, but he was certainly wary of the thing. I always suspected that he might have known on some level (thanks to LTT) that Callandor was not working right and was too dangerous to use.

That leaves the CK. And the fact that Rand did not use that against the shadowspawn is a problem all its own. I mean with the CK, he did not exactly need any fancy killer weaves. Demandred thinks that Rand could draw enough power from the CK to "melt continents". Rand could just have turned the entire blight into glass. No need for this complex weave then.

It is rather annoying actually that this was never even considered. I mean I can understand why he would not want to use it against the Seanchan and so on since there is such a large risk of collateral damage and mass death of innocent civilians. But there are no innocent civilians in the blight. Rand could have significantly reduced the trolloc population with the CK and saved hundreds of thousands of lives that will now die when the trollocs invade.

Why is it that Nynaeve for instance, with her worry for Lan never suggested: "Hey Rand! Why dont you blow up the blight with your fancy access key so that there won't be any trollocs left for Lan to die fighting"

Sigh.