The back plate is there because otherwise the Y-bearing mount plates are effectively cantilevered, hence in my opinion it's worth having something to reinforce these joints. However I agree that plates on the sides, as you suggest, would offer much better reinforcement, but this would increase the width of the Z-axis and loose some travel on Y, assuming the width of the Z-axis has already minimised. The plate certainly doesn't need to be the same thickness as the other parts, it was just convenient for the one in post #20 since I didn't have much of anything less than 20mm in stock, plus it was an easy way to support the motor mount.

Iwant1: How far are you with the design / build now?