They increase the base stats of all the weapons of the same type, and also activate their own often powerful skill(s) when you have 5 or more. As such, using SRs to get a full Axe Grid usually isn't worth it. But if you have five+ Axes in some Grid already *cough*Dark*cough* go ahead and slap that bitch in there.
The Axe isn't particularly good though, since it gives Exceed(++) (nice but not all that necessary) and Elaborate(++) (utter trash). Still, the increased base stats might make up for the loss in Assault.No. Soul weapons go into main slot.