Graendal performed her function by throwing Arad Doman into total chaos-- even Rand thinks the country is now beyond repair.

We saw how close the Last Battle was. There are multiple interpretations to this, but Moridin set up Graendal to be killed by placing the Choedan Kal that close. His main goal was to further destabilize the pattern in order to preserve the end goal of destroying the pattern completely.

Rand definitely destabilized the pattern by using so much balefire. Moridin set him up to that as well.

The Last Battle, according to Moridin is ultimately a battle between the Dark One and Rand. By throwing Rand into madness-- Moridin has already achieved his goal. Rand was well on the way to doing the Dark One's bidding on his own. In fact, Rand almost did it on Dragonmount when he talked about destroying the pattern.

But Moridin did not understand love, and that was his downfall. Because of love, Rand was able to come out of his darkness. I don't think Moridin could have predicted that.

Since Moridin only really cares about destroying the pattern and he was so close to achieving that goal with Rand-- he no longer had need of Graendal. The rest of the Forsaken were completely irrelevent. Moridin told Graendal to cause Rand 'pain of heart.' But I don't think Graendal realized that Moridin intended to use Graendal herself as a sacrifice to cause Rand that 'pain of heart.'

This is my interpretation, and it could be wrong-- but Moridin was VERY close to achieving his goal. Graendal, the Forsaken, and even the whole shadowspawn army would have been totally irrelevent if that goal had been achieved. It was wonderful risk to take, but it may not have payed off.