The flaw was either deliberate or it was unwittingly done but designed by the 'Pattern'. Considering it magnifies the taint alone, it is possible that no one knew of the flaw until the Breaking started. We also saw it in Rand's vision of the AOL alongside his banner, surrounded by Aes Sedai who were Foretelling. They obviously knew of its importance regarding the Dragon Reborn, which is why the warded it for him. I doubt its use was as trivial as to slay the unleashed Forsaken, though I do think that Callandor did provide a lesser benefit in that it forced Rand closer to his potential (although I think this was the principle purpose for the Eye of the World).

The flaw magnifies the taint. This may be a clue that Rand can use it to channel massive amounts of the TP with it. Lews Therin says that something has to touch the DO in order to seal him away, and that is ultimately what led to the taint i.e. saidin touched him. Therefore if Rand uses the TP to touch the Dark One and then uses saidin/saidar to make a perfect seal, it will allow him to seal the Bore perfectly. He still needs two women to balance the insanity it causes, and to provide the female half of the Power.

Using the TP seems to go against the new Rand and what he stands for, but there must be something underlying regarding his convergence with Moridin. Either its to allow him to draw on the TP, or as others think, for a soul swap.