There's no denying the final role of the BA was to be cannon fodder during the LB, though don't forget the Light's women aren't really better trained.

I think the destruction of the BA is a bigger blow than you believe. It's just that it's part of a larger picture of events surrounding the Tower that together form a "blow to the Shadow". It's not just the destruction of the BA, it's also the failure of the Reds to return with dark Asha'man, it's Egwene managing to avoid a bloodshed and reunite the Tower, it's Egwene managing to repel the attack of the Seanchan, and the Seanchan removing Elaida - and it's Egwene managing to remove Mesaana from the gameboard, and planting the seeds of an alliance between the groups of female channelers, likely a prelude to the reunion with the male Aes Sedai. This is all connected, providing a cleansed Aes Sedai organization for TG, one that given it looks like it will work with the male channellers during TG (it's not even impossible the two groups decide to merge soon), that it will be the magnet for the other groups of channellers to revolve around during the LB, and given it has double its pool of female channellers with the new novices alone, is a big blow to the Shadow. The Shadow had put tons and tons of efforts over centuries to destroy the Tower at the last minute before TG, so it would be too late for it to recover in time. It nearly succeeded, but it all ends with the opposite, a Tower that seems now to have become stronger than it has been for many centuries, with a few sisters of Forsaken strength.

Do you really believe the Shadow meant to leave the Tower so soon and this way? I very much doubt that. The departure of the BA was probably meant by the Shadow to be devastating = Mesaana most likely meant this move to be the removal of the key piece that brought the whole thing down at last, rendering the WT not much more than useless in the LB. It's very likely Mesaana meant to empty the Tower of its angreal and san'angreal, for one thing. The BA were the tools Mesaana used to split and divide the Tower. She very nearly managed to succeed at destroying the institution, dividing it so much there wasn't much they could have done during TG, especially with her puppet Amyrlin Elaida.

There's no doubt Mesaana was a much greater danger than the BA combined, but without her "children"/lapdogs Ishamael had nurtured for millenia inside the WT itself, and managed to keep hidden all this time (no sister was ever proven to be BA....), there wasn't much Mesaana would have accomplished. And the BA purge had to happen, or Egwene's reunited Tower would have been badly infiltrated during TG itself (or would have forced Rand to purge the WT of all its DF himself... as if Egwene and the AS would so easily have believed in his power to identify DF on sight...)

And losing so many of the women he no doubt planned to use for mixed gender circles to counter the Light's probably made Demandred swore when he got the news. Yeah, as far as we know the DF Asha'man are far more dangerous and "precious" than the BA in his arsenal, but Mesaana lost many assets along the way, and the purge made that much worse and the Shsdow have now lost a fair amount of its female channelers, while at the same time this gained the Light hundred and hundred of women who can be trusted not to be infiltrated DF, and who are now united and soon no doubt will "join" the forces of the Light more actively. Unless there are things we still don't know, Demandred is badly outnumbered in terms of channelers for the LB. The "male Aes Sedai" thanks to the attack in Tear are probably now far better trained in devastating battle weaves (including those a small group of channelers can use to destroy a 100,000 strong army of Trollocs and Myrddraal in minutes with minimal losses...) and in that circle, soon to be reunited with "the whole" mixed gender work is again common place and will soon spread to Egwene's forces, and the same from the WO with Perrin to the whole of them.

The whole string of events have not crippled Demandred, but he'll probably need to be much more at the top of his game as strategist and tactician during TG than he planned to require to win this, especially if Mat ends up being his counterpart for the Light, now that Rand has all of LTT's memories and can teach Mat the basics of using channellers in battle and let his "genius" do the rest. A great deal will depend on how well Demandred prepared for this, if he has prepared to face an enemy as dangerous as LTT was in the WOS or if he's been too lazy and overconfident (like his ally Semirhage...), thinking he'll face mostly pathetic female channelers too divided by Mesaana to work together and undertrained Asha'man the women hate and don't trust, and people Shadowspawn alone could defeat by their sheer numbers alone. The Cleansing no doubt made him expect the enemy could use mixed gender circles to a limited extent (I doubt the extent it's likely to be used was foreseen in anything but Demandred's worst nightmares...), but since WH or so, Demandred must have gotten one bad news after another. his failure to avert the Cleansing, his failure to find any opportunity to remove Moridin while his own position among the Chosen diminished, the failure to turn the Rebel AS against looking for a deal with the Asha'man, failure to infiltrate Asha'man in TV with the Red Ajah, the sudden re-appearance of WOS style mass destruction weaves that let small groups of channelers shred massive Shadowspawn armies, the rapid propagation of mixed gender work in Rand's inner circle and now the reunion of the inner circle with the rest of the Asha'man/AS at Merrilor, Semirhage's downfall, Mesaaana's downfall and the loss of a good chunk of the BA... and the Light discovering what a dreamspike is and does and managing to get it in a way he probably doesn't know about yet musn't please him one bit, when the same toy isolate the BT right now... At this point, Demandred must be dying for the battle to begin before his position worsens even more.

The loss of so many of the BA has certainly fragilized Demandred's position, and there was the fact Ishamael had organized them with a hidden hierarchy that just waited to come forward as their open leaders during TG and the BA purge has also destroyed that and left the BA as a weak and disorganized bunch... and without even a female Forsaken at their head Demandred could more or less trust and could use to teach these women, especially once the possibility she allied with Semirhage against him was out of the way and Mesaana needed him very badly.

That's not good, though I agree that without everything else happening happening around it, bringing about the WT as it will stand for TG, the BA purge isolated is probably not such a big blow to the Shadow, certainly not in the same league as the two other events I think qualify as real big blows (Cleansing and Epiphany). Many other events weakened the Shadow, but I see nothing so far that has had the impact of the Cleansing and Epiphany, not on their own and not even strung together to form greater "wholes".