In my opinion, the new design (screenshot) is impractical.
Before, even with three columns of widgets to choose from, it was a hassle dragging widgets from the Available or Inactive sections on the left column, to place it in a sidebar/footer on the right column, but now even more so.
I’m not seeing any advantages here, even for themes that have numerous widget areas, like Oxygen. (screenshot)
The Widgets Dash is one place, like the main in-site Dashboard, where I’d appreciate being able to choose the number of columns to display.