The idea about the X-axis is to improve stability and reliability.
The all-plastic solution I used previously was fine but the moving deck wasn't easy to locate with precision. The X-axis motor, actually an encoder-motor, was on the moving deck, making it big and heavy.
So basically, the X-axis motor should be stationary and the moving desk carry only the Y-axis motor.
To achieve such a goal, I performed various tests, using long metal axis for guiding and moving, and aluminium bars.