So normally I would agree if using the slot for support on the back side.
The front side is a different matter.
Most miter gauge bars need to be fully engaged in the TS miter slot to travel parallel to the blade. The further, out in front of the TS top, that the miter gauge face or fence is, the more it will wobble and or not be square to the blade. If the the miter bar is out on the improvised support it is likely going to be guiding a piece of wood that requires it to be out there. You need that slot to be accurately for that purpose.