Reminds me of the theories (which I and others supported) that Semirhage actually must have had a more clever plan than to simply try capturing Rand disguised as Tuon in KoD. The main reason was that we thought that a Top Forsaken like Semirhage wouldn't be that stupid. But we learned in TGS that Semirhage indeed failed greatly. Graendal wasn't even stupid in TGS, but the Semirhage-lesson taught me that no Forsaken is such intelligent as the "Graendal escaped"-followers believe now. She simply couldn't anticipate Rand's action.

Besides that, it makes more sense that Graendal's storyline was ended that way to concentrate on other things in the last two books, similar to Masema's, Semirhage's, Suroth's, Sevanna's, Galina's and maybe Elaida's conclusion.