Question about glass negative carriers

I was just staring at a nice Beseler 8346 carrier (glass, 6x6,
for the 45 enlarger) and the wheels started turning.

Why didn't/doesn't anyone make a glass carrier with the _top_
of the upper glass ground? Wouldn't this help diffuse the light,
sort of like turning an incandescent-based enlarger into a cold
light? The ground side of the glass should be far enought from
the plane of focus that it wouldn't show in the image, shouldn't
it? I suppose some illumination would be lost, but one could
always use a bigger bulb, or enjoy the additional time available
for dodging.

Is there some fatal flaw with this idea? Idle minds, you know...

Thanks.

Steve