I've only seen this on MR events. The smaller streamers use whatever table is at the venue, which these days is 95% a Diamond Pro-Am with a ball return. I assume its because MR has a lot more mics near the table than the other guys.
Put me down for the sound as well being the reason.
There were some early TAR Studio matches on a Pro-Am coin op, which is even louder than the standard pro-am. We'd hear the ball rolling in the mechanism and then popping out on every single shot.
Does anyone know how MR modifies the Pro-Am to be drop pocket? Do they stuff the pockets on site, or does Diamond special make them for them?