you can cut with xacto, but I think there would be a couple of inconveniences...an xacto requires that you pull the blade thru the material being cut while a rotary pushes down from the top to cut...I'd be afraid of pulling the fabric out of alighment with an exacto because of the pulling motion. The rulers would need to be completely slip-resistant to prevent fabric from moving with the exacto..while currently a ruler can slip when we use the rotary blade, the fabric usually doesn't. Safety...I'd be much more concerned with safety with the exacto...if it slips while cutting, the movement would have the blade coming towards you and with the rotary the movement is/should be away from you. I also prefer the protection on the rotary covers over the blade covers for the xacto.
All that being said, I prefer the rotary blades.