....the fact that tempering a sword is permanent, whereas the Fades' human-bled swords gradually lose their power after a while and have to be replaced. Personally I had always thought the thakan'dar stabbing-a-person-to-season-the-blade thing had more to do with the weapon absorbing the person's soul, which then gradually dissipates from the weapon thus requiring it to be renewed.

THREADJACK:
Either that, or it's a nice plot device to explain why there aren't a bajillion fades running around; the limits on their weaponry places limits on their battle numbers. Although, the info in the books doesn't necessarily add up; in one book, it's said that the Fades are upset and restless that there are so few humans at thakan'dar to make new swords with. This would imply that there are less Fades available for the armies, since they do not have adequate weaponry. However, as of KoD through ToM, there are easily half a million trollocs being gathered in the Blight (enough to throw away 100k of them in a battle with Rand at Algarin's manor, and another 100k at Maradon later), which implies that there must be a proportionate number of Fades to control them. Either there are indeed many more Fades than usual, enough to control the massive trolloc buildup, and the Fades complaining about the lack of humans were simply wrong and just kvetching; or the Fades who were complaining are right and in fact there are not enough of them to control the armies being built.