A lot of people said Verin was stupid to only mention the shadowspawn attack on Caemlyn in a letter that Mat never opened but I think it makes perfect sense.

The reason she compartmentalizes her intelligence reports among the different characters is that she know each one can only concentrate on a few things. Dividing the Tower's attention between its own affairs and Andor's (and presumably the blight-border) could mean defeat on both fronts. Mat and his army on the other hand are the perfect tools for this task. Being taveren ensures that the shadow doesn't turn the tables and win. Notice how Mat's taveren influence makes Olver open the letter when it needs to while still allowing Mat to save Moiraine.

If the Caemlyn gateway was destroyed this shadowspawn army could have cropped up anywhere in the Westland. Instead it will go to Caemlyn where there are light-forces to defeat it.