Because the examples of a weaker channeler actually defeating a stronger one in this story - certainly up to the point that RJ died - are virtually non-existent.

Egwene states that she could not hope to defeat Moghedien - the weakest of the Forsaken.

The fact is that strength has a much bigger influence on who would win a One Power duel - assassinations or unexpected attacks excluded - than you seem to give credit for.

And the strength list is important to me in the same way that it is important to know who is the higher level character between Elminster and Larloch, or who has the better 100m time between Usain Bolt and Asafa Powell.