You'd be wrong because Moridin mentionned this as an important matter to deal with before the LB. He said a lot depend on small detail like Isam's success to deal with the renegade in time.

It's obvious why, after you see Mashadar and the Taint annihilating each other. Shai'tan can't let the power of Shadar Logoth anywhere near SG, and he can't deal with him himself. Most likely Moridin or anything dealing with the TP can't either. Jordan's explanation is that the SL evil and the DO's evil cannot co-exist in the same space. With the Shadowspawn, it appears Fain is able to substitute the DO's touch on them (banish it, so to speak) by the SL touch (and this probably isn't new... there were early mentions of unnatural creatures tied to SL in legend. They pursued Caar One-Hand after he escaped SL, and eventually killed him).