I can see the benefits of going to a sort of mixed system -...

I can see the benefits of going to a sort of mixed system - where a majority or almost a majority of council is at large, but it is easier for people to start the process of running for council if they only have to run in their ward, and they can handle it all by just walking around and talking to their neighbors and knocking on doors. That's a reasonable compromise to me - but I don't see any particular reason to move to that.