Moridin predicted the exact path Rand would follow if he happened to channel the True Power.

Moridin knew Rand's mind better than anyone else due to their mental connection. I personally think Moridin predicted the path that Rand would follow-- a path leading to addiction to the True Power, despair, and utter darkness with no regard for the pattern anymore. Moridin would know this since he, himself, has walked this exact path. The Choedan Kal in dark Rand's hands could no good for the world, even if it was used to kill a Forsaken. The amount of damage Rand created was far more than Graendal would have ever created.

Moridin understood that a dark-Rand would be so hard that he would no longer have any regard for the consequences of his actions in preparing for the Last Battle. Rand began doing reckless things like balefiring a whole palace and he almost balefired his own father and the entire Seanchan army. Moridin understood that Rand might follow this path because Moridin, himself, had already followed this path. Moridin also understood that the Choedan Kal posed no real threat to the Dark One. As we began to learn in TGS, power is not the key to defeating the DO. The DO really doesn't care how much damage Rand can do with the CK if Rand is ultimately destroying the pattern in the process. Moridin left the CK with Rand so that Rand could do the Dark One's own work: the ultimate Fisher.

Rand ultimately realized something that Moridin could never know: the power of love. Moridin could not have predicted Rand's epiphany on Dragonmount. Rand realized that power was not the key to winning the Last Battle.

We will probably learn more in AMOL, but I actually think it is reasonable that Moridin/SH/the DO had schemed the possibility that Rand might draw on the True Power.