When I first installed Plasma I got confused as it seemed some applications open only on my second monitor. To fix that I enabled Window Management -> Window Behavior -> Active monitor follows mouse. I think it would be good to have that enabled by default.
Remember that there also is "Activate window on mouse hover" or something like that, which is disabled by default. So one thing would be changed by hovering the mouse, and one not. This might lead to some confusion.
But I personally agree with what you suggest.