I see no reason not to add this, it's a minimal change with massive uses that can't really confuse new Scratchers, as they would have to enable it and the x position could stay shown rounded in the sprite editor area where you can manually set position, direction, etc.

This allows for more advanced use of the screen, as many have said I often see the changing one pixel at a time, which isn't very smooth but with this it can be smooth.

Furthermore, Scratch already allows decimal x and y values, the displayed position is just rounded but if you use this script:

repeat (4)
change x by (0.3)

the sprite will move 1 pixel to the right.

In addition, like the OP stated this allots for more precise pen movement, and pen is a substantial corner piece of many projects, which this would improve.

Lastly, this won't cause problems in projects, because if it was like how post #4 stated, it would be an style or option on integer only values, which would not affect any past projects as other updates have.