I think it is still a huge blow to the Shadow losing 100's of channelers. Those would have been the dreadlords leading the Shadows Forces in the last battle, now there is a huge chunk of them just gone. We have saw what a small group of channelers can do to huge groups of regular troops (Rand and crew at the manor killing 100k+ trollocs with only a handful of channelers). Any side losings 100's of channelers at once (even mediocre ones) would suffer a huge setback that would take a long time to recover from.

Even though the black ajah did not die in the White Tower they were still eradicated permanently because the list with their names is still there and everyone knows who they are. The Tower is now whole again, and this is another huge blow to the shadow. People may mock the power of the White Tower, but that doesn't change the fact that 4-5 Forsaken thought it was important enough to infiltrate and try to control.