I can see where you're coming from and I agree with you for the most part that assault/exceed only weapons are not worth the brick unless its for a phantom weapon. However, I think there are a few exceptions that are worth mentioning. Merc axe is BY FAR the best weapon in water to brick right now because of water's lack of strong exceed weapons. It's so important to water grids at the moment that some water whales roughly measure strength based on how many mercs you have in your grid (not the most accurate method in the world and is a bit dated, but it just shows how crucial mercs are for water right now). Ukko axe and maybe Golden Staff are also notable since they are the hardest hitting exceed weapons for their respective elements as far as I know, but they are not nearly as important as merc is for water.
Draco eye weps will also be getting buffed some time down the road, so there's another reason to invest in mercs and ukkos.
Oh right, thanks for the heads up. I'll make sure to look into them. If you have any suggestions for additions to the list, they would be greatly appreciated.