Disclaimer: I’m no expert, this is just from my limited knowledge and experience. 
The dado is to prevent radial forces/load on the screws that mount the two plates together, this reduces the chance of screws from shearing, compared to a butt joint. I don’t think the cross brace will help prevent that nearly as much but would of course help forces in other directions with the additional rigidity.
And of course they’re plenty of joints on the Shapeoko that are merely a butt joint 