I don't know why it's not accepted either. My grandmother machine-bound every quilt she ever made as far as I can tell, so it's not THAT new of a concept.
I machine bind all of my quilts too, for the same reasons you do. I often use decorative stitches and I think when it's done well it adds to a quilt's appearance.
Maybe it IS acceptable at certain shows? I have only ever entered one quilt into a "show" but it was just the local fair. I did win a ribbon though! But I didn't get any sort of scorecard (just some nice comments scribbled on the back of my ID card that was pinned to the quilt) so I don't know if my machine binding knocked points off or not.